feat(bulk-edit): add bulkSelection store backed by SvelteSet

Module-singleton live accumulator: selection persists across pagination
and route changes within /documents and /enrich. Cleared on successful
bulk save or via Alles aufheben.

Refs #225

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

frontend/src/lib/stores/bulkSelection.svelte.ts

@@ -0,0 +1,36 @@
+import { SvelteSet } from 'svelte/reactivity';
+
+// Live accumulator. Selection persists across pagination and route changes
+// within /documents and /enrich. Cleared on successful bulk save or via
+// "Alles aufheben". The store is module-singleton — there is only ever one
+// bulk-edit selection per browser session.
+const selectedIds = new SvelteSet<string>();
+
+export const bulkSelectionStore = {
+	get ids(): SvelteSet<string> {
+		return selectedIds;
+	},
+	get size(): number {
+		return selectedIds.size;
+	},
+	has(id: string): boolean {
+		return selectedIds.has(id);
+	},
+	toggle(id: string): void {
+		if (selectedIds.has(id)) selectedIds.delete(id);
+		else selectedIds.add(id);
+	},
+	add(id: string): void {
+		selectedIds.add(id);
+	},
+	remove(id: string): void {
+		selectedIds.delete(id);
+	},
+	setAll(ids: Iterable<string>): void {
+		selectedIds.clear();
+		for (const id of ids) selectedIds.add(id);
+	},
+	clear(): void {
+		selectedIds.clear();
+	}
+};