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/css/style.css

@@ -398,6 +398,15 @@ body.theme-light .sidebar {
     margin-bottom: 8px;
 }
 
+/* Explanatory line under a control -- and where the import reports back. Not a
+   `label`, so it keeps sentence case and normal letter spacing. */
+.setting-note {
+    margin: 8px 0 0;
+    font-size: 12px;
+    line-height: 1.45;
+    color: var(--text-secondary);
+}
+
 .setting-label-row label {
     margin-bottom: 0;
 }