## Summary
Implements file explorer-style multi-selection functionality for media
assets in the AssetsSidebarTab component.
## Changes
### Multi-Selection Interactions
- **Normal click**: Single selection (clears previous, selects new)
- **Shift + click**: Range selection (from last selected to current)
- **Ctrl/Cmd + click**: Toggle individual selection
### State Management
- Added `assetSelectionStore` to manage selected asset IDs using Set
- Created `useAssetSelection` composable for selection logic and
keyboard state
### UI Enhancements
- Display selection count in footer (output tab only)
- Interactive selection count that shows "Deselect all" on hover
- Added bulk action buttons for download/delete (UI only)
### Translation Keys
Added new keys under `mediaAsset.selection`:
- `selectedCount`: "{count} selected"
- `deselectAll`: "Deselect all"
- `downloadSelected`: "Download"
- `deleteSelected`: "Delete"
## Test Plan
- [x] Open Assets sidebar tab
- [x] Switch to Generated tab
- [x] Test single selection with normal click
- [x] Test range selection with Shift + click
- [x] Test toggle selection with Ctrl/Cmd + click
- [x] Verify selection count updates correctly
- [x] Test hover interaction on selection count
- [x] Click "Deselect all" to clear selection
- [x] Test bulk action buttons (UI only)
## Notes
- Bulk download/delete functionality to be implemented in separate PR
- Selection UI currently only shows in output (Generated) tab
[screen-capture.webm](https://github.com/user-attachments/assets/740315bd-9254-4af3-a0be-10846d810d65)
