fix(bulk-upload): accessibility improvements and fetch comment

- BulkDropZone: link description <p> to drop zone region via aria-describedby
- UploadSaveBar: add explicit aria-valuenow/aria-valuemin/aria-valuemax to
  <progress> element for consistent screen reader support across browsers
- FileSwitcherStrip: add non-color error indicator (red !) to error chips so
  error state is not communicated by color alone (WCAG 1.4.1)
- BulkDocumentEditLayout: comment explaining why raw fetch is used instead of
  a SvelteKit form action (chunked FormData with per-chunk progress tracking)

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

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

@@ -102,6 +102,9 @@ $effect(() => {
 							>{i + 1}</span
 						>
 						<span class="max-w-[8rem] truncate" title={entry.title}>{entry.title}</span>
+						{#if entry.status === 'error'}
+							<span aria-hidden="true" class="ml-0.5 font-extrabold text-red-600">!</span>
+						{/if}
 					</button>
 					<button
 						type="button"