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/components/session-manager.tsx

@@ -250,7 +250,7 @@ export function SessionManager({
             >
               <Plus className="w-4 h-4 mr-1" />
               New
-              <span className="ml-1.5 px-1.5 py-0.5 text-[10px] font-mono rounded bg-white/20 text-white/80">
+              <span className="ml-1.5 px-1.5 py-0.5 text-[10px] font-mono rounded bg-primary-foreground/20 border border-primary-foreground/30 text-primary-foreground">
                 {ACTION_SHORTCUTS.newSession}
               </span>
             </Button>