♻️ refactor: extract Phase 1 hooks from sidebar (2187→2099 lines)

Extract 3 simple hooks with no UI dependencies:
- use-theme-preview.ts: Debounced theme preview on hover
- use-sidebar-auto-collapse.ts: Auto-collapse on small screens
- use-drag-and-drop.ts: Project reordering drag-and-drop

Benefits:
- Reduced sidebar.tsx by 88 lines (-4%)
- Improved testability (hooks can be tested in isolation)
- Removed unused imports (DragEndEvent, PointerSensor, useSensor, useSensors)
- Created hooks/ barrel export pattern

Next steps: Extract 10+ remaining hooks and 10+ UI sections to reach
target of 200-300 lines (current: 2099 lines, need to reduce ~1800 more)

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

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

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

@@ -0,0 +1,3 @@
+export { useThemePreview } from './use-theme-preview';
+export { useSidebarAutoCollapse } from './use-sidebar-auto-collapse';
+export { useDragAndDrop } from './use-drag-and-drop';