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;

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

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"

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

@@ -19,6 +19,9 @@ let {
 		<progress
 			value={chunkProgress.done}
 			max={chunkProgress.total}
+			aria-valuenow={chunkProgress.done}
+			aria-valuemin={0}
+			aria-valuemax={chunkProgress.total}
 			aria-label={m.bulk_upload_progress({ done: chunkProgress.done, total: chunkProgress.total })}
 			class="[&::-webkit-progress-bar]:bg-brand-sand mb-3 h-1 w-full rounded-full [&::-webkit-progress-bar]:rounded-full [&::-webkit-progress-value]:rounded-full [&::-webkit-progress-value]:bg-accent"
 		></progress>