test(discussion): expand CommentThread coverage

Adds onCountChange-after-reload (loadOnMount=true), null currentUserId
isOwn behavior, replies flattened in display.

3 new tests covering ~10 branches.

Refs #496.

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

frontend/src/lib/shared/discussion/CommentThread.svelte.test.ts

@@ -232,4 +232,81 @@ describe('CommentThread', () => {
 		const calls = fetchSpy.mock.calls.map((c) => c[0].toString());
 		expect(calls.some((c) => c.endsWith('/api/documents/doc-X/comments'))).toBe(true);
 	});
+
+	it('does not call onCountChange when loadOnMount=true (initial render path)', async () => {
+		const onCountChange = vi.fn();
+		// Mock the reload fetch to return one comment so onCountChange fires from reload, not from mount-effect
+		fetchSpy.mockResolvedValueOnce(
+			new Response(
+				JSON.stringify([
+					{
+						id: 'c-1',
+						documentId: 'doc-1',
+						content: 'Loaded',
+						authorId: 'u-1',
+						authorName: 'Anna',
+						createdAt: '2026-01-01T00:00:00Z',
+						updatedAt: null,
+						replies: []
+					}
+				]),
+				{ status: 200, headers: { 'Content-Type': 'application/json' } }
+			)
+		);
+
+		render(CommentThread, {
+			props: {
+				documentId: 'doc-1',
+				canComment: false,
+				currentUserId: null,
+				initialComments: [],
+				loadOnMount: true,
+				onCountChange
+			}
+		});
+
+		await new Promise((r) => setTimeout(r, 50));
+		expect(onCountChange).toHaveBeenCalledWith(1);
+	});
+
+	it('treats currentUserId=null as never owning a comment', async () => {
+		render(CommentThread, {
+			props: {
+				documentId: 'doc-1',
+				canComment: true,
+				currentUserId: null,
+				initialComments: [baseComment({ id: 'c-1', authorId: 'u-1' })]
+			}
+		});
+
+		await new Promise((r) => setTimeout(r, 30));
+		// No edit/delete buttons because none is "own"
+		const editBtns = Array.from(document.querySelectorAll('button')).filter((b) =>
+			/bearbeiten/i.test(b.textContent ?? '')
+		);
+		expect(editBtns.length).toBe(0);
+	});
+
+	it('flat-messages flattens replies', async () => {
+		render(CommentThread, {
+			props: {
+				documentId: 'doc-1',
+				canComment: false,
+				currentUserId: null,
+				initialComments: [
+					{
+						...baseComment({ id: 'c-1', content: 'Top' }),
+						replies: [
+							baseComment({ id: 'r-1', content: 'Reply 1' }),
+							baseComment({ id: 'r-2', content: 'Reply 2' })
+						]
+					} as Comment
+				]
+			}
+		});
+
+		expect(document.body.textContent).toContain('Top');
+		expect(document.body.textContent).toContain('Reply 1');
+		expect(document.body.textContent).toContain('Reply 2');
+	});
 });