feat(stats): add headword sentence search and rename related words

- Search by headword enabled by default; finds inflected variants (e.g. 知らない → 知らねえ)
- Add "Search by headword" toggle to switch back to exact text/title matching
- Rename "Similar Words" → "Related Seen Words" with tighter matching (same reading/shared kanji)
- ankiConnect.deck falls back to Yomitan mining deck when empty

changes/related-seen-words.md

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+type: changed
+area: stats
+
+- Renamed the vocabulary detail “Similar Words” section to “Related Seen Words” and tightened matches to same readings or shared kanji, avoiding noisy kana-suffix matches.

changes/stats-sentence-search-headword.md

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+type: added
+area: stats
+
+- Added headword-based sentence search by default, so searches like `知らない` can find tracked lines containing inflected variants such as `知らねえ` while exact sentence text searches still work.
+- Added a Search by headword toggle on the Stats search page for switching back to exact text/title matching.