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/stores/selection.ts

@@ -0,0 +1,65 @@
+import { writable, derived } from 'svelte/store';
+
+interface SelectionState {
+	active: boolean;
+	ids: Set<string>;
+}
+
+function createSelectionStore() {
+	const { subscribe, update, set } = writable<SelectionState>({
+		active: false,
+		ids: new Set(),
+	});
+
+	return {
+		subscribe,
+
+		enter() {
+			update((s) => ({ ...s, active: true }));
+		},
+
+		exit() {
+			set({ active: false, ids: new Set() });
+		},
+
+		toggle(id: string) {
+			update((s) => {
+				const ids = new Set(s.ids);
+				if (ids.has(id)) {
+					ids.delete(id);
+				} else {
+					ids.add(id);
+				}
+				// Exit selection mode automatically when last item is deselected
+				const active = ids.size > 0;
+				return { active, ids };
+			});
+		},
+
+		select(id: string) {
+			update((s) => {
+				const ids = new Set(s.ids);
+				ids.add(id);
+				return { active: true, ids };
+			});
+		},
+
+		deselect(id: string) {
+			update((s) => {
+				const ids = new Set(s.ids);
+				ids.delete(id);
+				const active = ids.size > 0;
+				return { active, ids };
+			});
+		},
+
+		clear() {
+			set({ active: false, ids: new Set() });
+		},
+	};
+}
+
+export const selectionStore = createSelectionStore();
+
+export const selectionCount = derived(selectionStore, ($s) => $s.ids.size);
+export const selectionActive = derived(selectionStore, ($s) => $s.active);