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/components/explorer-sidebar.tsx

@@ -381,8 +381,8 @@ const styles = StyleSheet.create((theme) => ({
     flexDirection: "row",
     alignItems: "center",
     justifyContent: "space-between",
-    paddingHorizontal: theme.spacing[3],
-    paddingVertical: theme.spacing[2],
+    paddingHorizontal: theme.spacing[2],
+    paddingVertical: theme.spacing[1],
     borderBottomWidth: 1,
     borderBottomColor: theme.colors.border,
   },