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diff --git a/packages/meshbay-hub/src/meshbay_hub/static/file-utils.js b/packages/meshbay-hub/src/meshbay_hub/static/file-utils.js
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+import * as downloads from './downloads.js';
+import * as platform from './platform.js';
+
+const FILE_ICONS = {
+  video: '\u{1F3AC}', audio: '\u{1F3B5}', image: '\u{1F5BC}',
+  document: '\u{1F4C4}', archive: '\u{1F4E6}', other: '\u{1F4CE}',
+};
+
+function formatSize(bytes) {
+  if (bytes < 1024) return bytes + ' B';
+  if (bytes < 1024 * 1024) return (bytes / 1024).toFixed(1) + ' KB';
+  if (bytes < 1024 * 1024 * 1024) return (bytes / (1024 * 1024)).toFixed(1) + ' MB';
+  return (bytes / (1024 * 1024 * 1024)).toFixed(2) + ' GB';
+}
+
+function formatDate(ts) {
+  return new Date(ts * 1000).toLocaleDateString(undefined, {
+    year: 'numeric', month: 'short', day: 'numeric',
+  });
+}
+
+const PREVIEWABLE_TEXT =
+  /\.(txt|md|json|csv|log|xml|yaml|yml|ini|conf|py|js|html|css|sh|c|h|java|rs|go|rb|toml)$/i;
+
+function canPreview(e) {
+  return ['image', 'video', 'document'].includes(e.type)
+    || PREVIEWABLE_TEXT.test(e.name);
+}
+
+const CHUNK_SIZE = 1024 * 1024;
+// Segments allowed in flight while there is room to put them. This is a window,
+// topped up as segments land, and not a debt released in one go: accumulating a
+// credit per append and handing the lot over when the buffer finally had room
+// sent 6 MB in a burst, overshot the target by a minute of film, and then said
+// nothing for the next forty-six seconds. Measured in Chrome against real
+// fragmented MP4. A stream that arrives in gulps has no margin for a network
+// that hesitates, and looks like a hang while it is quiet.
+const PIPELINE_WINDOW = 8;
+
+/**
+ * Open somewhere to write, honouring the user's download setting.
+ *
+ * Returns a target ({writable, name}), null for "no stream available — collect
+ * it and hand the browser a blob", or false for "the person dismissed the
+ * dialog", which is not an error and must not start a transfer.
+ */
+async function _openDownloadTarget(filename, size = 0, pickerOpts = {},
+                                   swSize = size) {
+  // On a desktop build this is the whole answer, and it comes first.
+  //
+  // The two browser paths below are both unavailable there — `showDirectoryPicker`
+  // does not exist, and Chromium refuses a service worker on a custom scheme —
+  // so without this the chain fell all the way through to its floor, which
+  // collects the file in the page and hands the browser a blob. A gigabyte of
+  // film meant a gigabyte of RAM, and a Save As dialog at the *end*.
+  if (platform.capabilities.nativeSave) {
+    try {
+      const native = await platform.nativeSave(
+        filename, { auto: downloads.getMode() === 'auto' });
+      // Null means the person dismissed the dialog, which is not an error and
+      // must not start a transfer.
+      return native || false;
+    } catch (err) {
+      console.warn('[MeshBay] native save failed:', platform.bridgeMessage(err));
+      return false;
+    }
+  }
+
+  try {
+    const target = await downloads.openTarget(filename);
+    if (target) return target;
+  } catch (err) {
+    console.warn('[MeshBay] download folder unusable:', err.message);
+  }
+
+  // No granted folder. A service worker can still hand the browser a stream to
+  // write, which is how this works at all in Firefox: the alternative there is
+  // to collect gigabytes in a tab. It goes to the browser's own download
+  // folder, without a dialog, which is what "save automatically" meant.
+  if (downloads.getMode() === 'auto') {
+    const streamed = await downloads.openStreamedDownload(filename, swSize);
+    if (streamed) return streamed;
+    // Nothing to stream to: small enough for memory, and no dialog.
+    if (size < downloads.BLOB_LIMIT) return null;
+  }
+
+  if (!window.showSaveFilePicker) return null;
+  try {
+    const handle = await window.showSaveFilePicker({
+      suggestedName: filename, ...pickerOpts,
+    });
+    return { writable: await handle.createWritable(), name: handle.name || filename };
+  } catch (err) {
+    if (err.name === 'AbortError') return false;
+    throw err;
+  }
+}
+
+/** The download of last resort, for browsers with no way to stream to disk. */
+function _saveBlob(blob, filename) {
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  URL.revokeObjectURL(url);
+}
+
+function _b64ToU8(b64) {
+  const bin = atob(b64);
+  const arr = new Uint8Array(bin.length);
+  for (let i = 0; i < bin.length; i++) arr[i] = bin.charCodeAt(i);
+  return arr;
+}
+
+async function pipelinedDownload(transport, gekKey, fileId, totalChunks, onChunk,
+                                 writable, signal) {
+  const results = writable ? null : new Array(totalChunks);
+  let nextSend = 0, nextRecv = 0;
+  const inflight = new Array(totalChunks);
+
+  const fire = () => {
+    while (nextSend < totalChunks && nextSend - nextRecv < PIPELINE_WINDOW) {
+      inflight[nextSend] = transport.fetchChunk(fileId, nextSend);
+      nextSend++;
+    }
+  };
+
+  fire();
+  while (nextRecv < totalChunks) {
+    if (signal && signal.aborted) {
+      const err = new Error('Cancelled');
+      err.name = 'AbortError';
+      throw err;
+    }
+    const chunkMsg = await inflight[nextRecv];
+    let plaintext;
+    if (gekKey && chunkMsg.ct) {
+      plaintext = await window.MeshBayCrypto.decryptChunkBin(
+        gekKey, fileId, nextRecv, chunkMsg.nonce, chunkMsg.ct);
+    } else if (gekKey && chunkMsg.ct_b64) {
+      plaintext = await window.MeshBayCrypto.decryptChunk(
+        gekKey, fileId, nextRecv, chunkMsg.nonce_b64, chunkMsg.ct_b64);
+    } else {
+      plaintext = _b64ToU8(chunkMsg.ct_b64 || chunkMsg.data_b64);
+    }
+    if (writable) {
+      await writable.write(plaintext);
+    } else {
+      results[nextRecv] = plaintext;
+    }
+    nextRecv++;
+    fire();
+    if (onChunk) onChunk(plaintext.byteLength, nextRecv, totalChunks);
+  }
+  return results;
+}
+
+/**
+ * Download one file through the transfers widget: picks a target, streams
+ * and decrypts it, and falls back to a blob when there is nowhere to stream
+ * to. Shared by the Files table/toolbar and the video/preview modals' own
+ * download button — both just want "get this entry to disk".
+ */
+async function downloadEntry(transfers, transport, gek, entry) {
+  const target = await _openDownloadTarget(entry.name, entry.size);
+  if (target === false) return;          // the picker was dismissed
+
+  const openRef = { url: null };
+  transfers.start({
+    kind: 'download', name: (target && target.name) || entry.name,
+    total: entry.size, transport,
+    open: target
+      ? (target.open || null)
+      : () => { if (openRef.url) window.open(openRef.url, '_blank'); },
+    run: async ({ signal, onProgress }) => {
+      const totalChunks = Math.ceil(entry.size / CHUNK_SIZE);
+      let done = 0;
+      const onChunk = (bytes) => { done += bytes; onProgress(done, entry.size); };
+
+      if (target) {
+        try {
+          await pipelinedDownload(transport, gek, entry.id, totalChunks,
+                                  onChunk, target.writable, signal);
+          await target.writable.close();
+        } catch (err) {
+          await target.writable.abort().catch(() => {});
+          throw err;
+        }
+      } else {
+        const chunks = await pipelinedDownload(
+          transport, gek, entry.id, totalChunks, onChunk, null, signal);
+        const blob = new Blob(chunks);
+        _saveBlob(blob, entry.name);
+        openRef.url = URL.createObjectURL(blob);
+      }
+    },
+  });
+}
+
+export {
+  FILE_ICONS,
+  formatSize, formatDate, PREVIEWABLE_TEXT, canPreview, CHUNK_SIZE,
+  _openDownloadTarget, _saveBlob, _b64ToU8, pipelinedDownload, downloadEntry,
+};