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/CommentThread.svelte

@@ -167,6 +167,9 @@ function cancelReply() {
 onMount(() => {
 	if (loadOnMount) {
 		reload();
+	} else {
+		const total = initialComments.reduce((s, c) => s + 1 + c.replies.length, 0);
+		onCountChange?.(total);
 	}
 });
 </script>
@@ -245,6 +248,24 @@ onMount(() => {
 {/snippet}
 
 <div class="space-y-4">
+	{#if comments.length === 0}
+		<div class="flex flex-col items-center gap-3 py-8 text-center">
+			<svg
+				class="h-10 w-10 text-ink-3"
+				viewBox="0 0 24 24"
+				fill="none"
+				stroke="currentColor"
+				stroke-width="1.5"
+			>
+				<path
+					stroke-linecap="round"
+					stroke-linejoin="round"
+					d="M2.25 12.76c0 1.6 1.123 2.994 2.707 3.227 1.087.16 2.185.283 3.293.369V21l4.076-4.076a1.526 1.526 0 0 1 1.037-.443 48.282 48.282 0 0 0 5.68-.494c1.584-.233 2.707-1.626 2.707-3.228V6.741c0-1.602-1.123-2.995-2.707-3.228A48.394 48.394 0 0 0 12 3c-2.392 0-4.744.175-7.043.513C3.373 3.746 2.25 5.14 2.25 6.741v6.018Z"
+				/>
+			</svg>
+			<p class="font-sans text-sm text-ink-3">{m.comment_empty_hint()}</p>
+		</div>
+	{/if}
 	{#each comments as thread, ti (thread.id)}
 		<div class={ti > 0 ? 'border-t border-line pt-4' : ''}>
 			<!-- Root comment -->