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>

packages/app/src/lib/send-rpc-request.ts

@@ -99,6 +99,7 @@ class RpcError extends Error {
  */
 const RESPONSE_TYPE_MAP: Record<string, SessionOutboundMessage["type"]> = {
   git_diff_request: "git_diff_response",
+  highlighted_diff_request: "highlighted_diff_response",
   file_explorer_request: "file_explorer_response",
   file_download_token_request: "file_download_token_response",
   git_repo_info_request: "git_repo_info_response",