feat(bulk-edit): add BulkSelectionBar and Alle-X-editieren fast path

- BulkSelectionBar component: sticky bottom bar shown only when canWrite
  and selection is non-empty. Buttons meet WCAG 44px touch targets and
  iOS safe-area inset is honoured.
- Bar mounted on /documents and /enrich.
- Alle X editieren button on /documents replaces the selection with
  every UUID matching the active filter (via /api/documents/ids) and
  jumps to /documents/bulk-edit.

Refs #225

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

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

@@ -99,3 +99,5 @@ const canWrite = $derived(data.canWrite);
 		</div>
 	{/if}
 </div>
+
+<BulkSelectionBar canWrite={canWrite} />