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/views/agent-output-modal.tsx

@@ -301,14 +301,14 @@ export function AgentOutputModal({
         </DialogHeader>
 
         {viewMode === "changes" ? (
-          <div className="flex-1 min-h-[400px] max-h-[60vh] overflow-hidden">
+          <div className="flex-1 min-h-[400px] max-h-[60vh] overflow-y-auto scrollbar-visible">
             {projectPath ? (
               <GitDiffPanel
                 projectPath={projectPath}
                 featureId={featureId}
                 compact={false}
                 useWorktrees={useWorktrees}
-                className="border-0 rounded-lg h-full"
+                className="border-0 rounded-lg"
               />
             ) : (
               <div className="flex items-center justify-center h-full text-muted-foreground">