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/changelog.md

@@ -6,6 +6,8 @@
 - Improved stats accuracy and scale handling with Yomitan token counts, full session timelines, known-word timeline fixes, cross-media vocabulary fixes, and clearer session charts.
 - Improved overlay/runtime stability with quieter macOS fullscreen recovery, reduced repeated loading OSD popups, and better frequency/noise handling for subtitle annotations.
 - Added launcher mpv-args passthrough plus Linux plugin wrapper-name fallback for packaged installs.
+- Added a hover-revealed ↗ button on Sessions tab rows to navigate directly to the anime media-detail view, with correct "Back to Sessions" back-navigation.
+- Excluded auxiliary-stem `そうだ` grammar tails (MeCab POS3 `助動詞語幹`) from subtitle annotation metadata so frequency, JLPT, and N+1 styling no longer bleed onto grammar-tail tokens.
 
 ## v0.6.5 (2026-03-15)
 - Seeded the AUR checkout with the repo `.SRCINFO` template before rewriting metadata so tagged releases do not depend on prior AUR state.