Persist stats exclusions in DB and fix word metrics filtering

- Stats vocabulary exclusions stored in `imm_stats_excluded_words` (schema v18); seeded from localStorage on first load
- Session, overview, trends, and library word metrics use filtered persisted occurrences with raw fallback
- Session known-word % chart uses filtered persisted totals as denominator for both known and total
- JLPT subtitle styling changed to underline-only; no longer overrides text color

stats/src/hooks/useSessions.ts

@@ -42,6 +42,7 @@ export function useSessions(limit = 50) {
 export interface KnownWordsTimelinePoint {
   linesSeen: number;
   knownWordsSeen: number;
+  totalWordsSeen: number;
 }
 
 export function useSessionDetail(sessionId: number | null) {