fix(bulk-upload): announce error chip status to screen readers

The ! indicator was aria-hidden with no sr-only fallback, making failed
uploads invisible to assistive technology. Added sr-only span with
bulk_file_error_chip_label before the visual indicator.

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

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

@@ -103,6 +103,7 @@ $effect(() => {
 						>
 						<span class="max-w-[8rem] truncate" title={entry.title}>{entry.title}</span>
 						{#if entry.status === 'error'}
+							<span class="sr-only">{m.bulk_file_error_chip_label()}</span>
 							<span aria-hidden="true" class="ml-0.5 font-extrabold text-red-600">!</span>
 						{/if}
 					</button>

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

@@ -78,6 +78,27 @@ describe('FileSwitcherStrip', () => {
 		expect(errBtn).not.toBeNull();
 	});
 
+	it('error chip contains a screen-reader-only error label', async () => {
+		const files: FileEntry[] = [
+			{
+				id: 'e1',
+				file: new File([''], 'bad.pdf'),
+				title: 'Bad file',
+				status: 'error',
+				previewUrl: ''
+			}
+		];
+		const { container } = render(FileSwitcherStrip, {
+			files,
+			activeId: 'e1',
+			onSelect: vi.fn(),
+			onRemove: vi.fn()
+		});
+		const errBtn = container.querySelector('[data-status="error"]');
+		const srOnly = errBtn?.querySelector('.sr-only');
+		expect(srOnly).not.toBeNull();
+	});
+
 	it('ArrowRight moves focus to next chip without leaving strip', async () => {
 		const files = makeFiles(3);
 		const { container } = render(FileSwitcherStrip, {