feat(stats): add library entry deletion and app-wide delete progress (#174)

changes/fix-stats-library-cover-after-relink.md

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+type: fixed
+area: stats
+
+- Relinking a title to a different AniList entry now updates its cover in the stats Library grid too. The detail view picked up the new art immediately while the grid kept the previous entry's image, so several unrelated titles could end up sharing one wrong cover. The library list is refetched after a relink, and cover responses now carry an ETag and revalidate instead of being cached for a day, so a stale image can no longer be served from the browser cache.

changes/stats-delete-library-entry.md

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+type: added
+area: stats
+
+- Added a "Delete Entry" action to the stats Library detail view. It removes the whole title in one step — every episode, session, subtitle line, rollup, cover art and vocabulary counts derived from them — and the title disappears from the Library grid. Previously only individual episodes and sessions could be deleted, so a mistakenly created entry had to be cleared episode by episode and still lingered in the library.
+- Deletion progress is now shown app-wide instead of only on Overview and Sessions. Every delete (session, session group, episode, and library entry) drives a sweeping bar at the top of the window plus a bottom-right status toast, and both stay visible across tab switches, detail views, and the in-app stats overlay window. The per-tab indicators previously went blank the moment you left the tab that started the delete.

changes/stats-delete-performance.md

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+type: fixed
+area: stats
+
+- Made deleting stats data much faster and stopped it stalling playback. Every delete used to recompute each affected word's totals by re-reading that word's whole occurrence history across the library, which meant one random read into the largest table per occurrence. On a 960-session library that cost 3.8s to delete a single session and over a minute to clear a 12-episode title, and because the stats server runs in the app process when the app owns it, that time was spent blocking mpv.
+- Word and kanji occurrences now store the subtitle line's timestamp alongside the count, so those totals are answered from a covering index instead of the subtitle-line table, and a delete only subtracts what it actually removed rather than recomputing untouched entries. Measured on the same 960-session library: deleting one session 3832ms to 190ms, a ten-session day group 6453ms to 433ms, one episode 5437ms to 263ms, and a whole 12-episode title 60556ms to 621ms.
+- Opening the Vocabulary tab no longer stalls either. It was computing "seen in N titles" for every word in the library before ordering by frequency and keeping the top 100, so the whole occurrence history was walked to produce one page. It now picks the page first and counts only those rows: 2136ms to 62ms on the same library.
+- The first launch after upgrading migrates the existing stats database in place (about 3s for a 288k-line library) and it grows roughly 20% from the added index. Vocabulary dates recorded before the upgrade keep working while the migration runs.