feat: complete remaining personalization and release gate (#139-#146)

[Dyu20705]

Summary

Completes the remaining scope of issues #133 → #146 in a single comprehensive PR for review:

Implemented Scope

• [Discoverability][1/3] Integrate Themes Tab into Settings Panel #140 — Settings/Themes Tab Integration: Tabbed modal dialog for general settings, built-in/custom theme management, and Japanese study goals.
• [Discoverability][2/3] Add Interactive Theme Onboarding and Customization Tour #141 — Onboarding Tour: Lightweight non-intrusive step-based tour pointing to key UI landmarks, with IndexedDB completion persistence, keyboard accessibility, and outside-click auto-dismissal.
• [Discoverability][3/3] Extend Command Palette with Theme and Japanese Learning Actions #142 — Command Palette Extensions: Extended command registry with 8 new commands (theme.import, theme.toggleDarkLight, settings.open, japanese.startReview, japanese.quickStudy, japanese.openKanjiDraw, japanese.setDailyGoal, editor.toggleToolbar).
• [Hardening][3/3] End-to-End User Journey Regression Suite and Contract Verification #146 — Release Gate E2E & Full Verification: Full user lifecycle E2E regression and contract coverage.

Pre-implemented and Verified Baseline

• [Epic] Japanese Workspace Enhancement — Gamification, Fluid Review, and Kanji Tools #133-[Japanese][5/5] Implement Daily Study Goal Setting and Progress Tracker #138: Japanese workspace, SRS flashcard review, gamification, Kanji drawing, quick study, daily goals.
• [Hardening][1/3] Mobile and Narrow Viewport Responsive Layout Hardening #144-[Hardening][2/3] Lazy Module Loading and Large Board Virtualization Optimization #145: Mobile responsive hardening, large board virtualization, and lazy Kanji dictionary asset loading.

Verification Evidence

• npm run test:content: PASS (3/3)
• npm run lint: PASS (0 errors, 0 warnings)
• npm run test:unit: PASS (331/331 tests)
• npm run test:integration: PASS (111/111 tests)
• npm run test:perf: PASS (6/6 budget tests)
• git diff --check: PASS (clean)

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.