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

backlog/tasks/task-329 - Keep-JLPT-subtitle-styling-underline-only.md

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+---
+id: TASK-329
+title: Keep JLPT subtitle styling underline-only
+status: Done
+assignee: []
+created_date: '2026-05-04 02:13'
+labels:
+  - bug
+  - renderer
+  - jlpt
+dependencies: []
+references:
+  - src/renderer/style.css
+  - src/renderer/subtitle-render.test.ts
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Fix subtitle token styling so JLPT metadata never changes token text color. JLPT should only render the level marker/underline affordance while known, n+1, name-match, and frequency colors retain priority.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 JLPT-only subtitle tokens do not set token text color.
+- [x] #2 JLPT level marker/underline still uses configured JLPT color.
+- [x] #3 Existing known, n+1, name-match, and frequency text colors remain unchanged.
+<!-- AC:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Changed subtitle JLPT styling from text color to underline decoration and updated renderer CSS regression coverage.
+
+Verification:
+- `bun test src/renderer/subtitle-render.test.ts`
+- `bunx prettier --check src/renderer/subtitle-render.test.ts src/renderer/style.css`
+- `bun run typecheck`
+
+Blocked:
+- `bun run test:fast` fails in existing dirty stats/session work: `recordSubtitleLine counts exact Yomitan tokens for session metrics` expects `tokensSeen` 4 but gets 3.
+<!-- SECTION:FINAL_SUMMARY:END -->