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/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;
+	}}
 />