Transcript viewer + search, in both UIs (3.x)

Surface the downloaded .vtt subtitles as a searchable, click-to-seek
transcript — a thing Tartube doesn't do at all.

Web (pure frontend): a 📄 button in the player opens a transcript pane
beside the video. It fetches the already-served .vtt, parses cues
client-side, and offers full-text search (highlighted), click-to-seek,
and a live highlight + auto-scroll of the current line as the video
plays. Reuses the chapters-pane layout; hidden on narrow screens.

Desktop (egui): a 📄 Transcript button on the detail panel opens a
floating window that reads the .vtt off disk (new `vtt` parser module),
shows the searchable cue list, and seeks the running mpv via its
JSON-IPC socket when you click a line.

New `src/vtt.rs`: a small, tolerant WebVTT/SRT cue parser (start time +
tag-stripped text, consecutive-duplicate collapse) with unit tests.

Closes the desktop/web parity gap for transcripts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.rs

@@ -30,6 +30,7 @@ mod stats;
 mod theme;
 mod tray;
 mod util;
+mod vtt;
 mod web;
 mod ytdlp_bin;