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/DocumentBottomPanel.svelte

@@ -49,6 +49,8 @@ type Props = {
 	height: number;
 	activeTab: Tab;
 	activeAnnotationId: string | null;
+	activeAnnotationPage: number | null;
+	onClearAnnotation?: () => void;
 	onAnnotationCommentCountChange?: (annotationId: string, count: number) => void;
 };
 
@@ -62,6 +64,8 @@ let {
 	height = $bindable(),
 	activeTab = $bindable(),
 	activeAnnotationId,
+	activeAnnotationPage,
+	onClearAnnotation,
 	onAnnotationCommentCountChange
 }: Props = $props();
 
@@ -182,10 +186,12 @@ const panelHeight = $derived(open ? height : MIN_HEIGHT);
 				<PanelDiscussion
 					documentId={doc.id}
 					activeAnnotationId={activeAnnotationId}
+					activeAnnotationPage={activeAnnotationPage}
 					initialComments={comments}
 					canComment={canComment}
 					currentUserId={currentUserId}
 					canAdmin={canAdmin}
+					onClearAnnotation={onClearAnnotation}
 					onAnnotationCommentCountChange={onAnnotationCommentCountChange}
 				/>
 			{:else if activeTab === 'history'}