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/BulkDocumentEditLayout.svelte

@@ -105,6 +105,9 @@ async function save() {
 			tagNames: tags.map((t) => t.name)
 		};
 		formData.append('metadata', new Blob([JSON.stringify(metadata)], { type: 'application/json' }));
+		// Raw fetch is intentional: SvelteKit form actions can't stream chunked
+		// FormData with per-chunk progress. Session cookie is sent automatically
+		// by the browser for same-origin requests.
 		const res = await fetch('/api/documents/quick-upload', { method: 'POST', body: formData });
 		if (!res.ok) {
 			hadErrors = true;