Add reusable tree component (src/renderer/components/ui/tree/) with:
- Slot-based icon customization
- DnD with position detection (before/after/center for containers)
- Inline rename with outline, no layout shift
- Multi-selection (single/range/toggle via consumer)
- Focus/highlight visual states
- External drop support
- Context menu via event emission

Migrate sidebar/folders to use UiTree as thin wrapper, removing
old TreeNode.vue, composables, and injection keys (-741 lines).