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

@@ -58,6 +58,7 @@ $effect(() => {
 });
 </script>
 
+<div aria-live="polite" aria-atomic="true" class="sr-only"></div>
 <div
 	data-testid="file-switcher-strip"
 	class="flex h-11 shrink-0 items-center gap-1 border-t border-line bg-pdf-ctrl px-2"
@@ -71,7 +72,7 @@ $effect(() => {
 	>
 
 	<div bind:this={trackEl} class="flex flex-1 gap-1 overflow-x-auto" style="scrollbar-width:none">
-		<ul bind:this={listEl} aria-live="polite" role="list" class="flex flex-row gap-1 py-1">
+		<ul bind:this={listEl} role="list" class="flex flex-row gap-1 py-1">
 			{#each files as entry, i (entry.id)}
 				<li role="listitem" class="inline-flex shrink-0 items-center">
 					<button
@@ -102,7 +103,7 @@ $effect(() => {
 					</button>
 					<button
 						type="button"
-						aria-label="Entfernen"
+						aria-label={m.bulk_remove_file()}
 						data-remove-id={entry.id}
 						onclick={() => onRemove(entry.id)}
 						class="ml-0.5 flex h-[44px] w-[44px] items-center justify-center text-base text-ink-3 hover:text-ink focus:outline-none focus-visible:ring-2 focus-visible:ring-accent"