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>

src/download_options.rs

@@ -72,6 +72,13 @@ pub struct DownloadOptions {
     /// to Tartube's `extra_cmd_string`.
     #[serde(default)]
     pub extra_args: Vec<String>,
+
+    /// Embed video comments into the info.json sidecar (yt-dlp
+    /// `--write-comments`). Off by default because comment download is slow
+    /// — yt-dlp paginates through thousands of replies for popular videos.
+    /// When on, the web UI's player modal exposes a Comments tab.
+    #[serde(default)]
+    pub fetch_comments: bool,
 }
 
 impl DownloadOptions {
@@ -119,6 +126,9 @@ impl DownloadOptions {
                 cmd.arg(arg);
             }
         }
+        if self.fetch_comments {
+            cmd.arg("--write-comments");
+        }
     }
 
     /// Deserialize from the JSON blob stored in `channel_options.options_json`.