fix(bulk-upload): i18n hardcoded strings in BulkDropZone and FileSwitcherStrip

- Add bulk_drop_desc, bulk_select_files, bulk_drop_zone_label, bulk_remove_file
  keys to de/en/es message files
- BulkDropZone: use m.bulk_drop_zone_label(), m.bulk_drop_desc(),
  m.bulk_select_files() — removes all hardcoded German
- FileSwitcherStrip: use m.bulk_remove_file() on × button; move aria-live
  from <ul> to a dedicated visually-hidden region above the strip (screen
  readers now announce changes without coupling the live region to the list)
- Spec: import FileEntry from component instead of re-declaring; use
  data-remove-id selector instead of hardcoded German aria-label

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

frontend/src/lib/components/document/FileSwitcherStrip.svelte.spec.ts

@@ -2,17 +2,10 @@ import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
 import FileSwitcherStrip from './FileSwitcherStrip.svelte';
+import type { FileEntry } from './FileSwitcherStrip.svelte';
 
 afterEach(cleanup);
 
-export interface FileEntry {
-	id: string;
-	file: File;
-	title: string;
-	status: 'idle' | 'error';
-	previewUrl: string;
-}
-
 function makeFiles(n: number): FileEntry[] {
 	return Array.from({ length: n }, (_, i) => ({
 		id: `id-${i}`,