a11y(dashboard): larger dismiss target + motion-reduce + sr-only PDF label

- UploadSuccessBanner dismiss button: 24×24 → 40×40 hit area (icon stays
  at 16px). Matches senior-first baseline Leonie flagged.
- DashboardNeedsMetadata chevron: adds motion-reduce:transition-none and
  motion-reduce:group-hover:translate-x-0 so users with prefers-reduced-
  motion do not see the hover translate.
- Row title prefixed with an sr-only "PDF: " span so assistive tech
  announces the document affordance alongside the title.

Addresses Leonie's review concerns #2, #3, and the sr-only nit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/UploadSuccessBanner.svelte

@@ -45,7 +45,7 @@ $effect(() => {
 		data-testid="upload-banner-close"
 		aria-label={m.upload_banner_close()}
 		onclick={onClose}
-		class="inline-flex h-6 w-6 items-center justify-center rounded-sm text-ink-3 hover:bg-ink/10 hover:text-ink"
+		class="inline-flex h-10 w-10 shrink-0 items-center justify-center rounded-sm text-ink-3 hover:bg-ink/10 hover:text-ink"
 	>
 		<svg
 			class="h-4 w-4"