fix(client): downloads stream to disk, and two rough edges on first run

**Downloads were going through RAM.** `_openDownloadTarget` tries a granted
folder, then a service worker, then its floor: collect the whole file in the
page and hand the browser a blob. Both of the first two are absent in the
desktop application — `showDirectoryPicker` does not exist, and Chromium refuses
a service worker on a custom scheme — so every download under 512 MB took the
floor. A gigabyte of film meant a gigabyte of RAM, and the only visible symptom
was a Save As dialog at the *end* rather than the start, which is what the
operator noticed and asked about.

The main process now streams to disk: it honours "save automatically" with a
folder chosen once and no dialog, never overwrites (a colliding name gets a
suffix), awaits each write so the renderer cannot outrun the disk and queue the
file in memory anyway, and unlinks a cancelled download rather than leaving a
truncated file that looks complete to whoever opens it next. Settings now offers
the native folder picker instead of saying downloads are unsupported.

Measured in the running application: the file on disk grows 256 KB → 512 KB →
768 KB → 1 MB as the chunks arrive, and an aborted download leaves nothing
behind.

**A permanent scrollbar on sign-in.** `.layout` and `.page-center` each reserved
`100vh - 52px`, and `.page-center` sits inside `main`'s 24px vertical padding —
so the page overflowed by exactly 48px at every window size. Found by measuring
in the app rather than reading the stylesheet: `scrollHeight` 819 against a 771
viewport, then the bottom edge of every element. The centring page brings its
own padding, so main's is dropped for it and the duplicated arithmetic goes
rather than growing a third term. Now `scrollHeight == innerHeight`, no
overflowing elements.

**The first-run screen was unstyled.** It used a class name I invented
(`auth-page`) that appears nowhere in the stylesheet, so it had no card and the
button sat against the input. It now uses the same `page-center` + `login-card`
markup as sign-in, which is where the 12px gap comes from. The sign-in link in
the nav is hidden until a hub is chosen — it led to a page that could not work.

809 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 23 insertions, 4 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/platform.js b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
index 994f558..372b666 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/platform.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/platform.js
@@ -176,13 +176,32 @@ export async function apiFetch(url, init) {
  * in `downloads.js`. Adding a native writer must not remove the three that
  * already work.
  */
-export async function nativeSave(suggestedName, size) {
+export async function nativeSave(suggestedName, { auto = true } = {}) {
   if (!bridge || !bridge.saveFile) return null;
-  return bridge.saveFile(suggestedName, size);
+  const sink = await bridge.saveFile(suggestedName, { auto });
+  if (!sink) return null;
+  // The shape every caller already expects from a download target: a `writable`
+  // with write/close/abort, and the name it was actually given on disk.
+  return {
+    name: sink.name,
+    writable: {
+      write: (bytes) => sink.write(bytes),
+      close: () => sink.close(),
+      abort: () => sink.abort(),
+    },
+  };
 }
 
+/** Where downloads go on a desktop build. Null in a browser. */
+export const folder = {
+  available: Boolean(bridge && bridge.folder),
+  async choose() { return bridge && bridge.folder ? bridge.folder.choose() : null; },
+  async get() { return bridge && bridge.folder ? bridge.folder.get() : null; },
+  async forget() { return bridge && bridge.folder ? bridge.folder.forget() : false; },
+};
+
 export default { isNative, hubBase, capabilities, secrets, nativeSave,
-                 apiFetch, device, bridgeMessage };
+                 apiFetch, device, bridgeMessage, folder };
 
 // Also a global, because `transport.js` is loaded as a classic script — it
 // predates the module graph and exposes `MeshBayTransport` the same way. The
@@ -191,5 +210,5 @@ export default { isNative, hubBase, capabilities, secrets, nativeSave,
 if (typeof window !== 'undefined') {
   window.MeshBayPlatform = { isNative, hubBase, capabilities, secrets,
                              nativeSave, apiFetch, device,
-                             bridgeMessage };
+                             bridgeMessage, folder };
 }