Import favorites from a Hot Tub backup

Settings gains a file picker that reads an exported Hot Tub database and
merges its favorites into this client's. The file never leaves the device:
sqlite.js is a small read-only reader -- header, schema, table b-trees,
record decoding, and the overflow pages that real rows here spill onto --
which is all it takes to walk one table, and avoids putting a wasm SQLite
behind a CDN fetch.

The two sides don't agree on what identifies a video. The app keys one by a
hash it computes locally (a 64-hex string); the server, and so this client,
keys it as something like "reddit-1rdudss". So the merge matches on
normalized URL: entries already saved here are left exactly as they are,
keeping the server id that makes a listing card's heart light up, and only
genuinely new videos are appended.

That means an imported favorite has no server id, so hearts now also match
by URL (`data-fav-url` on the card, feed slide and favorites bar). Without
it an imported favorite would look unsaved on its own card, and clicking
the heart would file a second copy of the same video.

Only the columns a favorite needs are read. `allFormats` is deliberately
left behind: it holds resolved, signed URLs, which is exactly what
favorites must not store (they expire -- see App.favorites.normalize).

Tests (scratchpad): the reader checked against Python's sqlite3 on a real
12MB backup -- table list, every table's row count, all 515 favorites with
their fields and order, and the 25 longest records byte-for-byte, which is
where a wrong overflow split shows up; and the Settings control driven
end-to-end, covering the merge, an existing favorite keeping its id, a
re-import adding nothing, and a listing card recognising an imported
favorite and unfavoriting it cleanly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QBDkEXP4htyXTCZUwMLphd

frontend/index.html

@@ -133,6 +133,17 @@
                 </div>
                 <div id="sources-list" class="sources-list"></div>
             </div>
+            <div class="sidebar-section">
+                <h4 class="sidebar-subtitle">Hot Tub Backup</h4>
+                <div class="setting-item">
+                    <label for="import-favorites-btn">Import Favorites</label>
+                    <input id="import-favorites-file" type="file" accept=".sqlite3,.sqlite,.db" hidden>
+                    <button id="import-favorites-btn" class="btn-secondary" type="button">Choose backup file…</button>
+                    <p id="import-favorites-status" class="setting-note" role="status" aria-live="polite">
+                        Reads the favorites out of an exported Hot Tub database. The file stays on this device.
+                    </p>
+                </div>
+            </div>
         </div>
     </aside>
 
@@ -191,6 +202,8 @@
     <script src="static/js/customPlayer.js"></script>
     <script src="static/js/player.js"></script>
     <script src="static/js/favorites.js"></script>
+    <script src="static/js/sqlite.js"></script>
+    <script src="static/js/hottubBackup.js"></script>
     <script src="static/js/videos.js"></script>
     <script src="static/js/feed.js"></script>
     <script src="static/js/ui.js"></script>