feat: implement file editor with syntax highlighting and editing

- Install react-simple-code-editor and prismjs dependencies
- Create language detection utility with 50+ file extensions
- Create SyntaxHighlighter component with Prism.js highlighting
- Create CodeEditor component with syntax-highlighted editing
- Create CommitDialog with diff preview and commit message
- Create FileEditor component integrating view/edit/commit flow
- Replace FileViewer with FileEditor in RepoWorkspace
- Add comprehensive CSS styles for editor, highlighter, and dialog
- Support keyboard shortcuts: Ctrl+E (toggle edit), Ctrl+S (save)
- Quality gates: typecheck ✓ lint ✓ build ✓

apps/web/package.json

@@ -11,10 +11,13 @@
     "test": "vitest run"
   },
   "dependencies": {
+    "@types/prismjs": "^1.26.6",
     "axios": "^1.6.0",
+    "prismjs": "^1.30.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "react-router-dom": "^6.20.0",
+    "react-simple-code-editor": "^0.14.1",
     "tailwindcss": "^3.3.0"
   },
   "devDependencies": {