feat(frontend): add discussion sub-tab navigation for annotation threads

Within the Diskussion panel tab, show two sub-tabs when an annotation is
active: «Diskussion» (document-level thread, with comment-count badge) and
«Annotation · Seite N» (annotation-specific thread).

Behaviour:
- Clicking an annotation auto-switches to the Annotation sub-tab
- Clicking the Diskussion sub-tab deselects the annotation and returns to
  the document thread
- Escape clears the active annotation (or collapses the panel if none)
- activeAnnotationPage is now lifted from PdfViewer → DocumentViewer →
  page → DocumentBottomPanel → PanelDiscussion so the tab label shows the
  correct page number

Closes #60
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/PdfViewer.svelte

@@ -9,12 +9,14 @@ let {
 	documentId = '',
 	annotateMode = $bindable(false),
 	activeAnnotationId = $bindable<string | null>(null),
+	activeAnnotationPage = $bindable<number | null>(null),
 	onAnnotationClick
 }: {
 	url: string;
 	documentId?: string;
 	annotateMode?: boolean;
 	activeAnnotationId?: string | null;
+	activeAnnotationPage?: number | null;
 	onAnnotationClick?: (id: string) => void;
 } = $props();
 
@@ -213,6 +215,7 @@ async function handleAnnotationDraw(rect: { x: number; y: number; width: number;
 			const created: Annotation = await res.json();
 			annotations = [...annotations, created];
 			activeAnnotationId = created.id;
+			activeAnnotationPage = created.pageNumber;
 			onAnnotationClick?.(created.id);
 		}
 	} catch {
@@ -236,6 +239,8 @@ async function handleAnnotationDelete(annotationId: string) {
 
 function handleAnnotationClick(id: string) {
 	activeAnnotationId = id;
+	const ann = annotations.find((a) => a.id === id);
+	activeAnnotationPage = ann?.pageNumber ?? null;
 	onAnnotationClick?.(id);
 }