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diff --git a/packages/meshbay-client/src/main.js b/packages/meshbay-client/src/main.js
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+/**
+ * MeshBay desktop client — the Electron main process.
+ *
+ * The reason this exists, stated once so nobody has to rediscover it: **the
+ * interface ships inside this package and loads from disk.** A shell that
+ * points a WebView at the hub's /app/ is a browser with a different icon and
+ * fixes nothing — the hub could still send whatever code it liked, which is
+ * finding T3. The hub is used for its API and for nothing else.
+ *
+ * What that does and does not buy is worth being honest about: it does not make
+ * the hub untrusted. A build downloaded from meshbay.org and signed with a key
+ * its operator holds relocates the trust rather than removing it. What changes
+ * is **detectability** — an attack has to ship as an artifact that can be
+ * hashed and compared, instead of being one HTTP response aimed at one person.
+ * That value is realised by reproducible builds and published hashes (18.7),
+ * not by the packaging format.
+ *
+ * See docs/desktop-client-v1.md §2 and §3.
+ */
+
+'use strict';
+
+const { app, BrowserWindow, dialog, ipcMain, protocol, safeStorage, shell } =
+  require('electron');
+const fs = require('node:fs');
+const fsp = require('node:fs/promises');
+const path = require('node:path');
+const { pathToFileURL } = require('node:url');
+
+const UI_DIR = path.join(__dirname, '..', 'ui');
+const SCHEME = 'app';
+
+// ── The scheme the interface is served from ─────────────────────────────────
+//
+// Not file://. Service workers, ES modules and IndexedDB all misbehave there,
+// and the streamed-download path needs a *controlled* page — a worker that is
+// merely active is not enough, which this codebase has already learned once.
+//
+// `secure` is what makes it a secure context, without which the worker refuses
+// to register and downloads break with no error at all. `standard` gives it a
+// real origin, so IndexedDB survives an update instead of being keyed to
+// something that moves.
+protocol.registerSchemesAsPrivileged([{
+  scheme: SCHEME,
+  privileges: {
+    standard: true,
+    secure: true,
+    supportFetchAPI: true,
+    stream: true,          // Range requests, for playing a film
+    corsEnabled: true,
+  },
+}]);
+
+/**
+ * Serve the packaged interface, and refuse to leave it.
+ *
+ * Every path is resolved and checked against the UI directory before anything
+ * is read: the renderer is the least trusted part of this process, and a
+ * traversal here would hand it the user's filesystem.
+ */
+function registerUiProtocol() {
+  protocol.handle(SCHEME, async (request) => {
+    const url = new URL(request.url);
+    const rel = decodeURIComponent(url.pathname).replace(/^\/+/, '') || 'index.html';
+    const target = path.resolve(UI_DIR, rel);
+    const root = path.resolve(UI_DIR);
+    if (target !== root && !target.startsWith(root + path.sep)) {
+      return new Response('Not found', { status: 404 });
+    }
+    try {
+      const body = await fsp.readFile(target);
+      return new Response(body, {
+        headers: { 'Content-Type': contentType(target) },
+      });
+    } catch {
+      return new Response('Not found', { status: 404 });
+    }
+  });
+}
+
+function contentType(file) {
+  const ext = path.extname(file).toLowerCase();
+  return {
+    '.html': 'text/html; charset=utf-8',
+    '.js': 'text/javascript; charset=utf-8',
+    '.mjs': 'text/javascript; charset=utf-8',
+    '.css': 'text/css; charset=utf-8',
+    '.json': 'application/json; charset=utf-8',
+    '.wasm': 'application/wasm',
+    '.svg': 'image/svg+xml',
+    '.png': 'image/png',
+    '.woff2': 'font/woff2',
+  }[ext] || 'application/octet-stream';
+}
+
+// ── Configuration ───────────────────────────────────────────────────────────
+
+function configPath() {
+  return path.join(app.getPath('userData'), 'config.json');
+}
+
+function readConfig() {
+  try {
+    return JSON.parse(fs.readFileSync(configPath(), 'utf8'));
+  } catch {
+    // No default hub. A client that picks its own is a client that can be
+    // pointed at one, so the first run asks and the answer is remembered.
+    return { hubBase: '', window: null };
+  }
+}
+
+function writeConfig(next) {
+  const dir = path.dirname(configPath());
+  fs.mkdirSync(dir, { recursive: true });
+  fs.writeFileSync(configPath(), JSON.stringify(next, null, 2), { mode: 0o600 });
+}
+
+let config = readConfig();
+
+// ── Secrets ─────────────────────────────────────────────────────────────────
+//
+// The OS keychain, through safeStorage. What it protects and what it does not
+// is reported rather than assumed: on Linux, safeStorage falls back to a fixed
+// key when no keyring is running — a headless session, a minimal desktop — and
+// it does so silently. Someone who believes the OS is holding their keys should
+// be told when it is not.
+
+function secretsFile() {
+  return path.join(app.getPath('userData'), 'secrets.bin');
+}
+
+function readSecrets() {
+  try {
+    const raw = fs.readFileSync(secretsFile());
+    if (!safeStorage.isEncryptionAvailable()) return {};
+    return JSON.parse(safeStorage.decryptString(raw));
+  } catch {
+    return {};
+  }
+}
+
+function writeSecrets(all) {
+  if (!safeStorage.isEncryptionAvailable()) {
+    throw new Error('No OS key storage is available on this system');
+  }
+  fs.mkdirSync(path.dirname(secretsFile()), { recursive: true });
+  fs.writeFileSync(secretsFile(), safeStorage.encryptString(JSON.stringify(all)),
+                   { mode: 0o600 });
+}
+
+function secretsBackend() {
+  if (!safeStorage.isEncryptionAvailable()) return 'unavailable';
+  if (process.platform !== 'linux') return process.platform === 'darwin'
+    ? 'keychain' : 'dpapi';
+  const backend = safeStorage.getSelectedStorageBackend
+    ? safeStorage.getSelectedStorageBackend() : 'unknown';
+  // "basic_text" is Electron's fixed-key fallback: encrypted on disk, but by a
+  // key that is not a secret. Named plainly so the interface can say so.
+  return backend === 'basic_text' ? 'unprotected_fallback' : backend;
+}
+
+// ── Window ──────────────────────────────────────────────────────────────────
+
+let mainWindow = null;
+
+function createWindow() {
+  const bounds = (config.window && config.window.width) ? config.window : {
+    width: 1200, height: 800,
+  };
+
+  mainWindow = new BrowserWindow({
+    ...bounds,
+    minWidth: 320,
+    show: false,
+    webPreferences: {
+      // The three that matter. `sandbox` keeps the Chromium renderer sandbox —
+      // the strongest one available, and the reason Electron is not the
+      // trade-off the earlier design recorded against "native". Without
+      // `contextIsolation` the preload's objects are reachable and mutable from
+      // page script, which would make the bridge below decorative.
+      sandbox: true,
+      contextIsolation: true,
+      nodeIntegration: false,
+      preload: path.join(__dirname, 'preload.js'),
+      // The hub address, handed over as a process argument rather than fetched.
+      // `platform.hubBase()` runs while the module graph is loading — before
+      // anything can await — so it has to be synchronous, and synchronous IPC
+      // would block the renderer on every call for a value that never changes
+      // within a run. Changing it restarts the window.
+      additionalArguments: [`--meshbay-hub=${config.hubBase || ''}`],
+      // The page is loaded over app:// and talks to the hub over https. Neither
+      // needs to reach the local filesystem.
+      webSecurity: true,
+    },
+  });
+
+  mainWindow.once('ready-to-show', () => mainWindow.show());
+
+  // The hub must never become the document origin. Anything that would navigate
+  // away from the packaged interface is refused, and an external link opens in
+  // the user's own browser rather than inside a window holding their keys.
+  const isOurs = (target) => {
+    try { return new URL(target).protocol === `${SCHEME}:`; } catch { return false; }
+  };
+  mainWindow.webContents.on('will-navigate', (event, target) => {
+    if (!isOurs(target)) event.preventDefault();
+  });
+  mainWindow.webContents.setWindowOpenHandler(({ url }) => {
+    if (/^https?:$/.test(new URL(url).protocol)) shell.openExternal(url);
+    return { action: 'deny' };
+  });
+  // Nothing in this application needs a camera, a microphone or a location.
+  mainWindow.webContents.session.setPermissionRequestHandler(
+    (_wc, _permission, callback) => callback(false));
+
+  mainWindow.on('close', () => {
+    if (!mainWindow.isMinimized() && !mainWindow.isFullScreen()) {
+      config = { ...config, window: mainWindow.getBounds() };
+      writeConfig(config);
+    }
+  });
+  mainWindow.on('closed', () => { mainWindow = null; });
+
+  mainWindow.loadURL(`${SCHEME}://meshbay/index.html`);
+}
+
+// ── Bridge ──────────────────────────────────────────────────────────────────
+//
+// Everything the interface may ask of this process, enumerated. A handler that
+// takes a path from the renderer and acts on it is the shape to avoid: the
+// renderer parses decrypted content from nodes, which is attacker-controlled
+// input, so it is treated as hostile even though it is our own code.
+
+function registerBridge() {
+  ipcMain.handle('hub:set', (_e, base) => {
+    const url = String(base || '').trim().replace(/\/+$/, '');
+    if (url && !/^https:\/\//.test(url) && !/^http:\/\/(localhost|127\.)/.test(url)) {
+      // http is allowed only to a loopback address, for someone running a hub
+      // on their own machine. Anywhere else it would put the session token on
+      // the wire in clear.
+      throw new Error('The hub address must be https');
+    }
+    config = { ...config, hubBase: url };
+    writeConfig(config);
+    // The renderer reads the address from a process argument, so the window has
+    // to be rebuilt for a change to take. Reloading in place would leave the
+    // interface talking to the old hub with no sign of it.
+    if (mainWindow) {
+      mainWindow.close();
+      createWindow();
+    }
+    return url;
+  });
+
+  ipcMain.handle('secrets:backend', () => secretsBackend());
+  ipcMain.handle('secrets:get', (_e, name) => readSecrets()[String(name)] ?? null);
+  ipcMain.handle('secrets:set', (_e, name, value) => {
+    const all = readSecrets();
+    all[String(name)] = String(value);
+    writeSecrets(all);
+    return true;
+  });
+  ipcMain.handle('secrets:clear', (_e, name) => {
+    const all = readSecrets();
+    delete all[String(name)];
+    writeSecrets(all);
+    return true;
+  });
+
+  // The renderer never names a path. It asks for a dialog; the user chooses;
+  // the main process holds the handle and the renderer only ever refers to it
+  // by an opaque id.
+  const sinks = new Map();
+  let sinkId = 0;
+
+  ipcMain.handle('save:begin', async (_e, suggestedName) => {
+    const result = await dialog.showSaveDialog(mainWindow, {
+      defaultPath: path.basename(String(suggestedName || 'download')),
+    });
+    if (result.canceled || !result.filePath) return null;
+    const id = String(++sinkId);
+    sinks.set(id, fs.createWriteStream(result.filePath));
+    return { id, name: path.basename(result.filePath) };
+  });
+
+  ipcMain.handle('save:write', async (_e, id, chunk) => {
+    const sink = sinks.get(String(id));
+    if (!sink) throw new Error('No such download');
+    await new Promise((resolve, reject) =>
+      sink.write(Buffer.from(chunk), (err) => (err ? reject(err) : resolve())));
+    return true;
+  });
+
+  ipcMain.handle('save:end', async (_e, id) => {
+    const sink = sinks.get(String(id));
+    if (!sink) return false;
+    sinks.delete(String(id));
+    await new Promise((resolve) => sink.end(resolve));
+    return true;
+  });
+}
+
+// ── Lifecycle ───────────────────────────────────────────────────────────────
+
+// One instance. Two would fight over the config file and the secrets blob, and
+// the second would look like the first had lost its state.
+if (!app.requestSingleInstanceLock()) {
+  app.quit();
+} else {
+  app.on('second-instance', () => {
+    if (mainWindow) {
+      if (mainWindow.isMinimized()) mainWindow.restore();
+      mainWindow.focus();
+    }
+  });
+
+  app.whenReady().then(() => {
+    registerUiProtocol();
+    registerBridge();
+    createWindow();
+    app.on('activate', () => {
+      if (BrowserWindow.getAllWindows().length === 0) createWindow();
+    });
+  });
+
+  app.on('window-all-closed', () => {
+    if (process.platform !== 'darwin') app.quit();
+  });
+}
+
+module.exports = { contentType, secretsBackend };