feat: Implement CodeMirror 6 file editor with tab support

Implement Phase 1 of the file editor & chat UI redesign:
- CodeMirror 6 integration with syntax highlighting
- Multi-file tab support with dirty state tracking
- Custom dark theme matching GitHub's color scheme
- Keyboard shortcuts (Ctrl+S to save, Ctrl+W to close tab)
- Mobile-responsive design with proper touch targets
- Fallback to basic textarea if CodeMirror fails to load

Technical details:
- Import map for ESM modules from node_modules
- Language support for JS, Python, HTML, CSS, JSON, Markdown
- Auto-initialization on DOM ready
- Global window.fileEditor instance for integration
- Serve node_modules at /claude/node_modules for import map

Files added:
- public/claude-ide/components/file-editor.js (main component)
- public/claude-ide/components/file-editor.css (responsive styles)

Files modified:
- public/claude-ide/index.html (import map, script tags)
- public/claude-ide/ide.js (updated loadFile function)
- server.js (serve node_modules for CodeMirror)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

server.js

@@ -189,6 +189,9 @@ app.get('/projects', (req, res) => {
 // Serve static files (must come after specific routes)
 app.use('/claude', express.static(path.join(__dirname, 'public')));
 
+// Serve node_modules for CodeMirror 6 import map
+app.use('/claude/node_modules', express.static(path.join(__dirname, 'node_modules')));
+
 // Login route
 app.post('/claude/api/login', (req, res) => {
   const { username, password } = req.body;