feat(#145): add DashboardResumeStrip component

- Component reads familienarchiv.lastVisited from localStorage and
  shows a 'Zuletzt geöffnet' link to the last-visited document
- Renders nothing when no localStorage entry exists
- Document detail page writes id+title to localStorage on mount

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -113,6 +113,14 @@ onMount(() => {
 
 	localStorageRestored = true;
 
+	// Track last-visited document for the dashboard resume strip
+	if (doc?.id) {
+		localStorage.setItem(
+			'familienarchiv.lastVisited',
+			JSON.stringify({ id: doc.id, title: doc.title ?? '' })
+		);
+	}
+
 	function onKeyDown(e: KeyboardEvent) {
 		if (e.key === 'Escape') {
 			if (activeAnnotationId) {