feat(stats): add media-detail navigation from Sessions rows; fix(tokenizer): exclude そうだ auxiliary-stem from annotations

- Added hover-revealed ↗ button on SessionRow that navigates to the
  anime media-detail view for the session's videoId
- Added `sessions` origin type to MediaDetailOrigin and
  openSessionsMediaDetail() / closeMediaDetail() handling so the
  back button returns correctly to the Sessions tab ("Back to Sessions")
- Wired onNavigateToMediaDetail down through SessionsTab → SessionRow
- Excluded tokens with MeCab POS3 = 助動詞語幹 (e.g. そうだ grammar tails)
  from subtitle annotation metadata so frequency, JLPT, and N+1 styling
  no longer apply to grammar-tail tokens
- Added annotation-stage unit test and end-to-end tokenizeSubtitle test
  for the そうだ exclusion path
- Updated docs-site changelog, immersion-tracking, and
  subtitle-annotations pages to reflect both changes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs-site/immersion-tracking.md

@@ -52,7 +52,7 @@ Watch time, sessions, words seen, and per-anime progress/pattern charts with con
 
 #### Sessions
 
-Expandable session history with new-word activity, cumulative totals, and pause/seek/card markers.
+Expandable session history with new-word activity, cumulative totals, and pause/seek/card markers. Each session row exposes a hover-revealed ↗ button that navigates to the anime media-detail view for that session; pressing the back button there returns to the Sessions tab.
 
 ![Stats Sessions](/screenshots/stats-sessions.png)