Add English level settings, improve content structuring and rendering

- Add english_level column to users table (CEFR with TOEIC mapping)
- Add UserController (GET/PATCH /api/users/me) and Settings page
- Enhance structuring prompts: sequential TOC, no summary sections,
  no content overlap, English expression extraction by CEFR level
- Remove sub-TOC analysis (caused content repetition), use simple
  per-section generation with truncation detection and continuation
- Fix CLOB truncation: explicit Clob-to-String conversion in repository
- Replace regex-based markdown rendering with react-markdown
- Add wallet renewal procedure to troubleshooting docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sundol-frontend/package-lock.json

@@ -13,7 +13,7 @@
         "next": "^15.3.1",
         "react": "^19.1.0",
         "react-dom": "^19.1.0",
-        "react-markdown": "^9.0.3",
+        "react-markdown": "^9.1.0",
         "zustand": "^5.0.3"
       },
       "devDependencies": {