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/messages/de.json

@@ -257,5 +257,6 @@
 	"doc_panel_tab_history": "Verlauf",
 	"doc_panel_annotate": "Annotieren",
 	"doc_panel_annotate_stop": "Fertig",
-	"doc_panel_annotation_thread_title": "Annotation"
+	"doc_panel_annotation_thread_title": "Annotation",
+	"doc_panel_discussion_annotation_tab": "Annotation · Seite {page}"
 }

frontend/messages/en.json

@@ -257,5 +257,6 @@
 	"doc_panel_tab_history": "History",
 	"doc_panel_annotate": "Annotate",
 	"doc_panel_annotate_stop": "Done",
-	"doc_panel_annotation_thread_title": "Annotation"
+	"doc_panel_annotation_thread_title": "Annotation",
+	"doc_panel_discussion_annotation_tab": "Annotation · Page {page}"
 }

frontend/messages/es.json

@@ -257,5 +257,6 @@
 	"doc_panel_tab_history": "Historial",
 	"doc_panel_annotate": "Anotar",
 	"doc_panel_annotate_stop": "Listo",
-	"doc_panel_annotation_thread_title": "Anotación"
+	"doc_panel_annotation_thread_title": "Anotación",
+	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}"
 }

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'}

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}

frontend/src/lib/components/PanelDiscussion.svelte

@@ -24,33 +24,98 @@ type Comment = {
 type Props = {
 	documentId: string;
 	activeAnnotationId: string | null;
+	activeAnnotationPage: number | null;
 	initialComments: Comment[];
 	canComment: boolean;
 	currentUserId: string | null;
 	canAdmin: boolean;
+	onClearAnnotation?: () => void;
 	onAnnotationCommentCountChange?: (annotationId: string, count: number) => void;
 };
 
 let {
 	documentId,
 	activeAnnotationId,
+	activeAnnotationPage,
 	initialComments,
 	canComment,
 	currentUserId,
 	canAdmin,
+	onClearAnnotation,
 	onAnnotationCommentCountChange
 }: Props = $props();
+
+// Sub-tab within the discussion panel: 'document' or 'annotation'
+type DiscussionTab = 'document' | 'annotation';
+let activeSubTab = $state<DiscussionTab>('document');
+
+// Track document-level comment count for badge.
+// CommentThread calls onCountChange immediately on mount with the accurate total.
+let docCommentCount = $state(0);
+
+// When an annotation becomes active, switch to the annotation sub-tab automatically.
+$effect(() => {
+	if (activeAnnotationId) {
+		activeSubTab = 'annotation';
+	} else {
+		activeSubTab = 'document';
+	}
+});
+
+function selectDocumentTab() {
+	activeSubTab = 'document';
+	onClearAnnotation?.();
+}
+
+function selectAnnotationTab() {
+	activeSubTab = 'annotation';
+}
 </script>
 
-<div class="space-y-8 p-6">
-	<!-- Annotation thread (shown when an annotation is active) -->
+<div class="flex h-full flex-col">
+	<!-- Sub-tab bar (only shown when annotation is active) -->
 	{#if activeAnnotationId}
-		<div>
-			<h4
-				class="mb-3 border-b border-brand-sand pb-2 font-sans text-xs font-bold tracking-widest text-brand-navy uppercase"
+		<div class="flex shrink-0 border-b border-brand-sand/70 bg-gray-50 px-4">
+			<button
+				onclick={selectDocumentTab}
+				class="mr-1 px-3 py-2 font-sans text-xs font-medium transition-colors {activeSubTab === 'document'
+					? 'border-b-2 border-brand-navy text-brand-navy'
+					: 'text-gray-400 hover:text-brand-navy'}"
 			>
-				{m.doc_panel_annotation_thread_title()}
-			</h4>
+				{m.doc_panel_tab_discussion()}
+				{#if docCommentCount > 0}
+					<span
+						class="ml-1 inline-flex h-4 min-w-4 items-center justify-center rounded-full bg-brand-mint px-1 font-mono text-[10px] text-brand-navy"
+					>
+						{docCommentCount}
+					</span>
+				{/if}
+			</button>
+			<button
+				onclick={selectAnnotationTab}
+				class="px-3 py-2 font-sans text-xs font-medium transition-colors {activeSubTab === 'annotation'
+					? 'border-b-2 border-brand-navy text-brand-navy'
+					: 'text-gray-400 hover:text-brand-navy'}"
+			>
+				{m.doc_panel_discussion_annotation_tab({ page: String(activeAnnotationPage ?? '?') })}
+			</button>
+		</div>
+	{/if}
+
+	<!-- Content area -->
+	<div class="flex-1 overflow-y-auto p-6">
+		{#if !activeAnnotationId || activeSubTab === 'document'}
+			<!-- Document-level thread -->
+			<CommentThread
+				documentId={documentId}
+				initialComments={initialComments}
+				canComment={canComment}
+				currentUserId={currentUserId}
+				canAdmin={canAdmin}
+				onCountChange={(count) => (docCommentCount = count)}
+			/>
+		{:else}
+			<!-- Annotation-level thread -->
 			{#key activeAnnotationId}
 				<CommentThread
 					documentId={documentId}
@@ -62,24 +127,6 @@ let {
 					onCountChange={(count) => onAnnotationCommentCountChange?.(activeAnnotationId, count)}
 				/>
 			{/key}
-		</div>
-	{/if}
-
-	<!-- General document discussion -->
-	<div>
-		{#if activeAnnotationId}
-			<h4
-				class="mb-3 border-b border-brand-sand pb-2 font-sans text-xs font-bold tracking-widest text-brand-navy uppercase"
-			>
-				{m.comment_section_title()}
-			</h4>
 		{/if}
-		<CommentThread
-			documentId={documentId}
-			initialComments={initialComments}
-			canComment={canComment}
-			currentUserId={currentUserId}
-			canAdmin={canAdmin}
-		/>
 	</div>
 </div>

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);
 }
 

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

@@ -56,6 +56,7 @@ async function loadFile(id: string) {
 
 let annotateMode = $state(false);
 let activeAnnotationId = $state<string | null>(null);
+let activeAnnotationPage = $state<number | null>(null);
 
 // When an annotation is clicked, open the Diskussion tab.
 $effect(() => {
@@ -97,6 +98,19 @@ onMount(() => {
 	}
 
 	localStorageRestored = true;
+
+	function onKeyDown(e: KeyboardEvent) {
+		if (e.key === 'Escape') {
+			if (activeAnnotationId) {
+				activeAnnotationId = null;
+				activeAnnotationPage = null;
+			} else if (panelOpen) {
+				panelOpen = false;
+			}
+		}
+	}
+	document.addEventListener('keydown', onKeyDown);
+	return () => document.removeEventListener('keydown', onKeyDown);
 });
 
 // Persist panel state whenever it changes (after initial restore).
@@ -129,6 +143,7 @@ $effect(() => {
 			error={fileError}
 			bind:annotateMode={annotateMode}
 			bind:activeAnnotationId={activeAnnotationId}
+			bind:activeAnnotationPage={activeAnnotationPage}
 			onAnnotationClick={(id) => {
 				activeAnnotationId = id;
 			}}
@@ -146,4 +161,9 @@ $effect(() => {
 	bind:height={panelHeight}
 	bind:activeTab={activeTab}
 	activeAnnotationId={activeAnnotationId}
+	activeAnnotationPage={activeAnnotationPage}
+	onClearAnnotation={() => {
+		activeAnnotationId = null;
+		activeAnnotationPage = null;
+	}}
 />