feat(ui): add per-project themes and fix scrolling/styling issues

- Add per-project theme support with theme selector in sidebar
- Fix file diffs scrolling in agent output modal with proper overflow handling
- Improve cursor styling across all interactive elements (buttons, tabs, checkboxes)
- Enhance hotkey styling to use consistent theme colors
- Fix Kanban board flash/refresh issues with React.memo optimizations
- Update tab component for better theme integration with proper active states

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

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

app/src/lib/electron.ts

@@ -1354,6 +1354,7 @@ export interface Project {
   name: string;
   path: string;
   lastOpened?: string;
+  theme?: string; // Per-project theme override (uses ThemeMode from app-store)
 }
 
 export interface TrashedProject extends Project {