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/platform/assets/composables/useAssetSelectionStore.ts

@@ -0,0 +1,81 @@
+import { defineStore } from 'pinia'
+import { computed, ref } from 'vue'
+
+export const useAssetSelectionStore = defineStore('assetSelection', () => {
+  // State
+  const selectedAssetIds = ref<Set<string>>(new Set())
+  const lastSelectedIndex = ref<number>(-1)
+
+  // Getters
+  const selectedCount = computed(() => selectedAssetIds.value.size)
+  const hasSelection = computed(() => selectedAssetIds.value.size > 0)
+  const selectedIdsArray = computed(() => Array.from(selectedAssetIds.value))
+
+  // Actions
+  function addToSelection(assetId: string) {
+    selectedAssetIds.value.add(assetId)
+  }
+
+  function removeFromSelection(assetId: string) {
+    selectedAssetIds.value.delete(assetId)
+  }
+
+  function setSelection(assetIds: string[]) {
+    // Only update if there's an actual change to prevent unnecessary re-renders
+    const newSet = new Set(assetIds)
+    if (
+      newSet.size !== selectedAssetIds.value.size ||
+      !assetIds.every((id) => selectedAssetIds.value.has(id))
+    ) {
+      selectedAssetIds.value = newSet
+    }
+  }
+
+  function clearSelection() {
+    selectedAssetIds.value.clear()
+    lastSelectedIndex.value = -1
+  }
+
+  function toggleSelection(assetId: string) {
+    if (isSelected(assetId)) {
+      removeFromSelection(assetId)
+    } else {
+      addToSelection(assetId)
+    }
+  }
+
+  function isSelected(assetId: string): boolean {
+    return selectedAssetIds.value.has(assetId)
+  }
+
+  function setLastSelectedIndex(index: number) {
+    lastSelectedIndex.value = index
+  }
+
+  // Reset function for cleanup
+  function reset() {
+    selectedAssetIds.value.clear()
+    lastSelectedIndex.value = -1
+  }
+
+  return {
+    // State
+    selectedAssetIds: computed(() => selectedAssetIds.value),
+    lastSelectedIndex: computed(() => lastSelectedIndex.value),
+
+    // Getters
+    selectedCount,
+    hasSelection,
+    selectedIdsArray,
+
+    // Actions
+    addToSelection,
+    removeFromSelection,
+    setSelection,
+    clearSelection,
+    toggleSelection,
+    isSelected,
+    setLastSelectedIndex,
+    reset
+  }
+})