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/lib/components/layout/Header.svelte

@@ -11,6 +11,7 @@
 		onOrderToggle: () => void;
 		onFilterToggle: () => void;
 		onUpload?: () => void;
+		onTrash?: () => void;
 	}
 
 	let {
@@ -22,6 +23,7 @@
 		onOrderToggle,
 		onFilterToggle,
 		onUpload,
+		onTrash,
 	}: Props = $props();
 </script>
 
@@ -43,6 +45,14 @@
 		</button>
 	{/if}
 
+	{#if onTrash}
+		<button class="icon-btn trash-btn" onclick={onTrash} title="Trash">
+			<svg width="15" height="15" viewBox="0 0 15 15" fill="none" aria-hidden="true">
+				<path d="M2 4h11M5 4V2.5h5V4M5.5 7v4.5M9.5 7v4.5M3 4l.8 9h7.4l.8-9" stroke="currentColor" stroke-width="1.6" stroke-linecap="round" stroke-linejoin="round"/>
+			</svg>
+		</button>
+	{/if}
+
 	<div class="controls">
 		<select
 			class="sort-select"