feat(dashboard): add code editor for raw JSON settings editing

- Add GET/PUT /api/file endpoints for generic file access with security validation
- Add GET /api/files endpoint to list editable JSON files in ~/.ccs/
- Create CodeEditor component with JSON syntax highlighting (prism-react-renderer)
- Add "Raw JSON" tab to SettingsDialog for direct JSON editing
- Support conflict detection, atomic writes, and automatic backups
- Lazy load editor to minimize initial bundle impact (~31KB gzipped)

Closes #73

ui/package.json

@@ -34,11 +34,13 @@
     "clsx": "^2.1.1",
     "date-fns": "^4.1.0",
     "lucide-react": "^0.556.0",
+    "prism-react-renderer": "^2.4.1",
     "react": "^19.2.0",
     "react-day-picker": "^9.12.0",
     "react-dom": "^19.2.0",
     "react-hook-form": "^7.68.0",
     "react-router-dom": "^7.10.1",
+    "react-simple-code-editor": "^0.14.1",
     "recharts": "^2.12.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",