feat(git-diff): server-side syntax highlighting with full file context

- Move syntax highlighting from client to server for proper AST context
- Add highlighted_diff_request/response message types
- Create diff-highlighter and syntax-highlighter utils on server
- Fix TSX dialect to use "ts jsx" for proper TypeScript parsing
- Redesign diff UI: card-style file sections, GitHub Dark theme colors
- Reduce visual clutter: smaller padding, subtle backgrounds

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

package-lock.json

@@ -21320,6 +21320,14 @@
       "dependencies": {
         "@ai-sdk/openai": "^2.0.52",
         "@deepgram/sdk": "^3.4.0",
+        "@lezer/common": "^1.5.0",
+        "@lezer/css": "^1.3.0",
+        "@lezer/highlight": "^1.2.3",
+        "@lezer/html": "^1.3.13",
+        "@lezer/javascript": "^1.5.4",
+        "@lezer/json": "^1.0.3",
+        "@lezer/markdown": "^1.6.2",
+        "@lezer/python": "^1.1.18",
         "@modelcontextprotocol/sdk": "^1.20.1",
         "@openrouter/ai-sdk-provider": "^1.2.0",
         "ai": "^5.0.76",