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/BulkDropZone.svelte

@@ -13,6 +13,7 @@ let isDragging = $state(false);
 <div
 	role="region"
 	aria-label={m.bulk_drop_zone_label()}
+	aria-describedby="bulk-drop-desc"
 	data-testid="bulk-drop-zone"
 	class="flex flex-1 flex-col items-center justify-center p-6"
 	ondragover={(e) => {
@@ -54,7 +55,7 @@ let isDragging = $state(false);
 		<p class="font-serif text-base font-bold text-ink">{m.bulk_drop_hint()}</p>
 
 		<!-- Sub description -->
-		<p class="text-sm leading-relaxed text-ink-2">{m.bulk_drop_desc()}</p>
+		<p id="bulk-drop-desc" class="text-sm leading-relaxed text-ink-2">{m.bulk_drop_desc()}</p>
 
 		<!-- CTA button -->
 		<label