fix(document): polish PDF error state — warning icon, 44px target, warmer copy

Address the remaining UI/UX polish: add a warning-triangle icon so the
failure is signalled by shape, not colour alone (WCAG 1.4.1); give the
recovery download link a full 44px tap/focus target (inline-flex
min-h-[44px]); and soften the message copy in de/en/es.

Addresses re-review: Leonie (colour-only, undersized link, copy warmth).

Refs #708

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/viewer/PdfViewer.svelte

@@ -178,12 +178,27 @@ function handleAnnotationClick(id: string) {
 		role="alert"
 		class="flex h-full w-full flex-col items-center justify-center gap-3 bg-pdf-bg px-4 text-center text-ink-3"
 	>
+		<!-- A shape, not colour alone, signals the warning (WCAG 1.4.1). -->
+		<svg
+			class="h-10 w-10 text-red-400"
+			viewBox="0 0 24 24"
+			fill="none"
+			stroke="currentColor"
+			stroke-width="2"
+			aria-hidden="true"
+		>
+			<path
+				stroke-linecap="round"
+				stroke-linejoin="round"
+				d="M12 9v3.75m-9.303 3.376c-.866 1.5.217 3.374 1.948 3.374h14.71c1.73 0 2.813-1.874 1.948-3.374L13.949 3.378c-.866-1.5-3.032-1.5-3.898 0L2.697 16.126zM12 15.75h.007v.008H12v-.008z"
+			/>
+		</svg>
 		<p class="font-sans text-base text-red-400">{m.doc_render_failed()}</p>
 		<a
 			href={url}
 			target="_blank"
 			rel="noopener noreferrer"
-			class="inline-block py-2 font-sans text-sm text-primary underline hover:text-ink-2"
+			class="inline-flex min-h-[44px] items-center px-3 py-2 font-sans text-sm text-primary underline hover:text-ink-2"
 		>
 			{m.doc_download_link()}
 		</a>