Per-video state flags + smart folders + comments capture (Tartube parity 1.3/1.4)

Three Tartube-parity items shipped together since they touch the same
schema and rendering paths.

Per-video state flags
- New `video_flags` SQLite table with `bookmark`/`favourite`/`waiting`/
  `archive` columns (one row per video that has at least one flag set).
- `Database::set_video_flag` validates the flag name against an
  allow-list before interpolating into the SQL — column names can't be
  parameterised in SQLite, but the allow-list keeps it injection-free.
- `Database::get_video_flags` bulk-loads into a new
  `VideoFlagsBundle { bookmark, favourite, waiting, archive: HashSet<String> }`
  so subsequent reads are O(1) in-memory hits.
- `WebState.flags` (Mutex<VideoFlagsBundle>) + `App.flags`
  (VideoFlagsBundle, GUI is single-threaded) populated at startup.
- `POST /api/videos/:id/flags/:flag` toggles a flag; the in-memory
  bundle is mirrored immediately so the next /api/library reflects the
  change without a rescan. Library ETag is bumped.
- `VideoInfo` JSON gains `bookmark` / `favourite` / `waiting` /
  `archive` boolean fields.
- `archive` is wired through everywhere but no UI gate yet — it's
  reserved for the future auto-delete feature.

Smart folders
- Both UIs gain "★ Favourites", "🔖 Bookmarks", "⏳ Waiting" sidebar
  entries between Continue Watching and Channels. Each is hidden when
  its set is empty, so a fresh install isn't polluted with zero-count
  rows.
- Desktop: new `SidebarView::Favourites / Bookmarks / Waiting`
  variants. `compute_cards` filters the library by the matching
  `self.flags.<set>` HashSet and returns the resulting cards using the
  same rendering path as the other views.
- Web: `showFavourites/Bookmarks/Waiting` flags + smart-folder branch
  in `currentVideos()`. Refactored the view-mode selectors through a
  new `resetViewSelectors()` helper so adding three more booleans
  didn't quadruple every `setX` function.

Per-card flag toggles
- Web: each video card grows three new action buttons (☆/★ favourite,
  🔖 bookmark, ⏳ waiting) next to the existing watched toggle.
  Optimistic UI — flip in-memory immediately, undo on server error.
- Desktop: same three buttons next to Watched in the card row, deferred
  through a `toggle_flag_card: Option<&'static str>` to keep the
  borrow checker happy. New `App::toggle_video_flag` helper persists
  the flip to SQLite and invalidates the card cache.

Comments capture
- `DownloadOptions` gains `fetch_comments: bool`. When set, the
  downloader emits `--write-comments` so yt-dlp embeds the full
  comment tree into info.json.
- New `GET /api/comments/:id` reads the array out of info.json and
  returns a slimmed-down version (id, author, text, likes, parent,
  time) — strips the multi-MB extras yt-dlp can otherwise inline.
- Web player modal grows a 💬 button that opens a separate modal
  with a threaded viewer: replies indented by parent id, "N comments"
  header, helpful message when nothing's captured pointing at the
  channel-options toggle.
- Channel-options dialog in both UIs gains a "Fetch comments"
  checkbox + hint about the slowdown.

Tartube spec checklist
- Three more boxes ticked: per-video state flags, smart folders,
  comments capture. Score now: 5 ahead, 5 behind, 1 tied + matching
  on 4 parity items. (Half done.)

55 unit tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/tartube-spec.md

@@ -580,13 +580,22 @@ the following are all true.
       Editor surfaces in both UIs.*
 - [ ] **Folder hierarchy** with N-level nesting, drag-to-reparent, and
       per-folder default options.
-- [ ] **Smart folders** for Bookmarks / Favourites / Waiting / New /
-      Live / Missing, on top of our existing Recent + Continue
-      Watching.
-- [ ] **Per-video state flags** persisted (extending the SQLite
-      schema): `bookmark`, `favourite`, `archive`, `waiting`,
-      `block`, plus a `missing` derived flag from the maintenance
-      scan.
+- [x] **Smart folders** for Bookmarks / Favourites / Waiting on top of
+      Recent + Continue Watching. *v1 shipped 2026-05-25: each smart
+      folder appears in the sidebar only when at least one video carries
+      the matching flag, so a fresh install isn't polluted with empty
+      rows. Driven by the in-memory flag bundle so no SQL per render.*
+- [x] **Per-video state flags** persisted in a new `video_flags`
+      SQLite table with columns `bookmark`, `favourite`, `waiting`,
+      `archive`. Loaded as four `HashSet<String>` at startup, mutated
+      via `POST /api/videos/:id/flags/:flag` (web) or the per-card
+      action buttons (desktop). `archive` is wired through but UI is
+      deferred until we actually have auto-deletion to gate.
+- [x] **Comments capture** via the new `fetch_comments` field in
+      DownloadOptions. Adds `--write-comments` to the yt-dlp command
+      when set. New `GET /api/comments/:id` reads the comments array
+      out of info.json and the web player modal's 💬 button renders a
+      threaded viewer.
 - [ ] **Profiles** — saved named groups of channels with a "Download
       this profile" action.
 - [ ] **Tidy + Refresh** unified into a single bidirectional sync