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/DocumentViewer.svelte

@@ -15,6 +15,7 @@ type Props = {
 	error: string;
 	annotateMode: boolean;
 	activeAnnotationId: string | null;
+	activeAnnotationPage: number | null;
 	onAnnotationClick: (id: string) => void;
 };
 
@@ -25,6 +26,7 @@ let {
 	error,
 	annotateMode = $bindable(),
 	activeAnnotationId = $bindable(),
+	activeAnnotationPage = $bindable(),
 	onAnnotationClick
 }: Props = $props();
 </script>
@@ -79,6 +81,7 @@ let {
 			documentId={doc.id}
 			bind:annotateMode={annotateMode}
 			bind:activeAnnotationId={activeAnnotationId}
+			bind:activeAnnotationPage={activeAnnotationPage}
 			onAnnotationClick={onAnnotationClick}
 		/>
 	{:else if fileUrl}