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.spec.ts

@@ -0,0 +1,70 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import CommentThread from './CommentThread.svelte';
+import type { Comment } from '$lib/types';
+
+afterEach(() => {
+	cleanup();
+	vi.unstubAllGlobals();
+});
+
+function makeComment(id: string, content = 'Hello'): Comment {
+	return {
+		id,
+		authorId: 'user-1',
+		authorName: 'Alice',
+		content,
+		createdAt: new Date().toISOString(),
+		updatedAt: new Date().toISOString(),
+		replies: []
+	};
+}
+
+const baseProps = {
+	documentId: 'doc-1',
+	canComment: true,
+	currentUserId: 'user-1',
+	canAdmin: false
+};
+
+describe('CommentThread – empty state', () => {
+	it('shows empty state hint when there are no comments', async () => {
+		render(CommentThread, { ...baseProps, initialComments: [] });
+		await expect
+			.element(page.getByText('Noch keine Kommentare – starte die Diskussion!'))
+			.toBeInTheDocument();
+	});
+
+	it('does not show empty state hint when comments exist', async () => {
+		render(CommentThread, { ...baseProps, initialComments: [makeComment('c-1')] });
+		await expect
+			.element(page.getByText('Noch keine Kommentare – starte die Diskussion!'))
+			.not.toBeInTheDocument();
+	});
+});
+
+describe('CommentThread – onCountChange', () => {
+	it('calls onCountChange with initial SSR count on mount', async () => {
+		const onCountChange = vi.fn();
+		render(CommentThread, {
+			...baseProps,
+			initialComments: [makeComment('c-1'), makeComment('c-2')],
+			onCountChange
+		});
+		expect(onCountChange).toHaveBeenCalledWith(2);
+	});
+
+	it('calls onCountChange with 0 when no initial comments', async () => {
+		const onCountChange = vi.fn();
+		render(CommentThread, { ...baseProps, initialComments: [], onCountChange });
+		expect(onCountChange).toHaveBeenCalledWith(0);
+	});
+
+	it('counts replies in the total', async () => {
+		const onCountChange = vi.fn();
+		const comment = { ...makeComment('c-1'), replies: [makeComment('r-1') as never] };
+		render(CommentThread, { ...baseProps, initialComments: [comment], onCountChange });
+		expect(onCountChange).toHaveBeenCalledWith(2);
+	});
+});