feat(frontend): implement trash view with restore and permanent delete

- New /files/trash page: same grid as files view, deleted files only
- Tap selects (no detail page for deleted files), long-press drag-selects
- Trash selection bar: Restore (bulk) and Delete permanently (bulk, confirmed)
- Trash icon added to files header, navigates to /files/trash
- Mock: MOCK_TRASH with 6 pre-seeded files; bulk/delete now moves to trash;
  handlers for POST /files/{id}/restore and DELETE /files/{id}/permanent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/files/+page.svelte

@@ -242,6 +242,7 @@
 		onOrderToggle={() => fileSorting.toggleOrder()}
 		onFilterToggle={() => (filterOpen = !filterOpen)}
 		onUpload={() => uploader?.open()}
+		onTrash={() => goto('/files/trash')}
 	/>
 
 	{#if filterOpen}