feat(overlay): Anki maturity-based known-word highlighting

Color known-word subtitle highlights by Anki card maturity (new,
learning, young, mature) like asbplayer (#171). Notes are classified
server-side with Anki search filters (prop:ivl, is:learn) during the
known-word cache refresh, so no per-card data is fetched. A word's tier
is its most mature matching card/note, with the same reading-aware
matching as boolean known-word lookups.

- known-word cache v4 state persists per-note tiers; lifecycle key only
  gains the maturity field while enabled so existing caches survive
- ankiConnect.knownWords.maturityEnabled + matureThresholdDays (21)
- subtitleStyle.knownWordMaturityColors with catppuccin defaults
- word-maturity-<tier> class rides on word-known so hover/selection
  rules keep applying; falls back to knownWordColor without tier data
- runtime toggle subtitle.annotation.knownWords.maturityEnabled

src/config/definitions/defaults-subtitle.ts

@@ -32,6 +32,12 @@ export const SUBTITLE_DEFAULT_CONFIG: Pick<ResolvedConfig, 'subtitleStyle' | 'su
     backdropFilter: 'blur(6px)',
     nPlusOneColor: '#c6a0f6',
     knownWordColor: '#a6da95',
+    knownWordMaturityColors: {
+      new: '#ee99a0',
+      learning: '#b7bdf8',
+      young: '#91d7e3',
+      mature: '#a6da95',
+    },
     jlptColors: {
       N1: '#ed8796',
       N2: '#f5a97f',