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>