feat: Add multi-select support for media assets (#6256)

## 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)

src/locales/en/main.json

@@ -1779,6 +1779,8 @@
   "mediaAsset": {
     "deleteAssetTitle": "Delete this asset?",
     "deleteAssetDescription": "This asset will be permanently removed.",
+    "deleteSelectedTitle": "Delete selected assets?",
+    "deleteSelectedDescription": "{count} asset(s) will be permanently removed.",
     "assetDeletedSuccessfully": "Asset deleted successfully",
     "deletingImportedFilesCloudOnly": "Deleting imported files is only supported in cloud version",
     "failedToDeleteAsset": "Failed to delete asset",
@@ -1787,6 +1789,16 @@
       "jobIdCopyFailed": "Failed to copy Job ID",
       "copied": "Copied",
       "error": "Error"
+    },
+    "selection": {
+      "selectedCount": "Assets Selected: {count}",
+      "deselectAll": "Deselect all",
+      "downloadSelected": "Download",
+      "deleteSelected": "Delete",
+      "downloadStarted": "Downloading {count} files...",
+      "downloadsStarted": "Started downloading {count} file(s)",
+      "assetsDeletedSuccessfully": "{count} asset(s) deleted successfully",
+      "failedToDeleteAssets": "Failed to delete selected assets"
     }
   },
   "actionbar": {