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

stats/src/components/vocabulary/WordDetailPanel.tsx

@@ -301,7 +301,7 @@ export function WordDetailPanel({
                 {data.similarWords.length > 0 && (
                   <section>
                     <h3 className="text-xs font-semibold uppercase tracking-wide text-ctp-overlay1 mb-2">
-                      Similar Words
+                      Related Seen Words
                     </h3>
                     <div className="flex flex-wrap gap-1.5">
                       {data.similarWords.map((sw) => (