feat(#81): improve discussion discoverability

- Add comment count badge on the Discussion tab (seeded from SSR, updated live)
- Add 'Diskussion starten' nudge above collapsed panel when no comments exist
- Add empty state hint with speech-bubble icon inside the discussion panel
- Fix CommentThread to fire onCountChange with SSR-seeded count on mount
- Add tests for all three behaviours in CommentThread and DocumentBottomPanel

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

frontend/src/lib/components/PanelDiscussion.svelte

@@ -8,9 +8,11 @@ type Props = {
 	canComment: boolean;
 	currentUserId: string | null;
 	canAdmin: boolean;
+	onCountChange?: (count: number) => void;
 };
 
-let { documentId, initialComments, canComment, currentUserId, canAdmin }: Props = $props();
+let { documentId, initialComments, canComment, currentUserId, canAdmin, onCountChange }: Props =
+	$props();
 </script>
 
 <div class="flex-1 overflow-y-auto p-6">
@@ -20,5 +22,6 @@ let { documentId, initialComments, canComment, currentUserId, canAdmin }: Props
 		canComment={canComment}
 		currentUserId={currentUserId}
 		canAdmin={canAdmin}
+		onCountChange={onCountChange}
 	/>
 </div>