feat(frontend): implement file selection with long-press, shift+click, and touch drag

- selection.ts: store with select/deselect/toggle/enter/exit, derived count and active
- FileCard: long-press (400ms) enters selection mode, shows check overlay, blocks context menu
- Header: Select/Cancel button toggles selection mode
- SelectionBar: floating bar above navbar with count, Edit tags, Add to pool, Delete
- Shift+click range-selects between last and current index (desktop)
- Touch drag-to-select/deselect after long-press; non-passive touchmove blocks scroll only during drag

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

frontend/src/lib/components/layout/Header.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
-	import type { FileSortField, SortOrder } from '$lib/stores/sorting';
+	import type { SortOrder } from '$lib/stores/sorting';
+	import { selectionStore, selectionActive } from '$lib/stores/selection';
 
 	interface Props {
 		sortOptions: { value: string; label: string }[];
@@ -23,6 +24,14 @@
 </script>
 
 <header>
+	<button
+		class="select-btn"
+		class:active={$selectionActive}
+		onclick={() => ($selectionActive ? selectionStore.exit() : selectionStore.enter())}
+	>
+		{$selectionActive ? 'Cancel' : 'Select'}
+	</button>
+
 	<div class="controls">
 		<select
 			class="sort-select"
@@ -70,6 +79,29 @@
 		flex-shrink: 0;
 	}
 
+	.select-btn {
+		height: 30px;
+		padding: 0 12px;
+		border-radius: 6px;
+		border: 1px solid color-mix(in srgb, var(--color-accent) 30%, transparent);
+		background-color: var(--color-bg-elevated);
+		color: var(--color-text-muted);
+		font-size: 0.85rem;
+		font-family: inherit;
+		cursor: pointer;
+	}
+
+	.select-btn:hover {
+		color: var(--color-text-primary);
+		border-color: var(--color-accent);
+	}
+
+	.select-btn.active {
+		background-color: color-mix(in srgb, var(--color-accent) 25%, var(--color-bg-elevated));
+		color: var(--color-accent);
+		border-color: var(--color-accent);
+	}
+
 	.controls {
 		display: flex;
 		align-items: center;