♻️ refactor: implement Phase 1 folder-pattern compliance

- Rename App.tsx to app.tsx (kebab-case naming convention)
- Add barrel exports (index.ts) for src/hooks/
- Add barrel exports (index.ts) for src/components/dialogs/
- Add barrel exports (index.ts) for src/components/layout/
- Update renderer.tsx import to use lowercase app.tsx

This is Phase 1 of folder-pattern.md compliance: establishing proper
file naming conventions and barrel export patterns.

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/ui/src/components/dialogs/index.ts

@@ -0,0 +1,2 @@
+export { BoardBackgroundModal } from './board-background-modal';
+export { FileBrowserDialog } from './file-browser-dialog';

apps/ui/src/components/layout/index.ts

@@ -0,0 +1 @@
+export { Sidebar } from './sidebar';