fix(chronik): split Für-dich row markup — Dismiss is sibling of link, not nested

HTML5 forbids interactive content (<button>, <a>, <input>...) as descendants
of <a>. The original <a href=…><button>✓</button></a> markup triggered two
concrete bugs flagged by Felix, Nora, and Leonie in PR #288 review:

- Browsers inconsistently route the nested click: on some engines the
  stopPropagation() still bubbles, and the user navigates into the document
  instead of dismissing.
- The senior audience (60+) tap-selects with a slight drag, and the OS
  treats the interaction as anchor vs. button inconsistently — a
  reproducible usability failure Leonie has seen in testing before.

Refactor to the Option-C layout from issue #285 comment #3573: outer <li>
flex container, <a> wrapping avatar + body + time, <button> as a sibling.
Independent focus stops, invalid-HTML gone, no behavioural regression.

A new spec locks the invariant: `dismiss.closest('a')` must be null.

Part of #285, address PR #288 review.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/chronik/ChronikFuerDichBox.svelte

@@ -18,7 +18,6 @@ function verb(type: NotificationItem['type'], actor: string): string {
 }
 
 function href(n: NotificationItem): string {
-	// Link to the specific comment/reference within the document.
 	return `/documents/${n.documentId}?commentId=${n.referenceId}`;
 }
 </script>
@@ -78,10 +77,12 @@ function href(n: NotificationItem): string {
 
 		<ul role="list" class="flex flex-col gap-2">
 			{#each unread as n (n.id)}
-				<li>
+				<li
+					class="fade-in group flex items-start gap-3 rounded-sm p-2 transition-colors hover:bg-canvas"
+				>
 					<a
 						href={href(n)}
-						class="fade-in group flex items-start gap-3 rounded-sm p-2 transition-colors hover:bg-canvas focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:outline-none"
+						class="flex min-w-0 flex-1 items-start gap-3 rounded-sm focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:outline-none"
 					>
 						<span
 							aria-hidden="true"
@@ -97,30 +98,26 @@ function href(n: NotificationItem): string {
 								{relativeTime(n.createdAt)}
 							</p>
 						</div>
-						<button
-							type="button"
-							data-testid="chronik-fuerdich-dismiss"
-							aria-label={m.chronik_mark_read_aria()}
-							onclick={(e) => {
-								e.preventDefault();
-								e.stopPropagation();
-								onMarkRead(n);
-							}}
-							class="mt-0.5 shrink-0 rounded-sm p-1 text-ink-3 transition-colors hover:bg-muted hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:outline-none"
-						>
-							<svg
-								xmlns="http://www.w3.org/2000/svg"
-								class="h-4 w-4"
-								fill="none"
-								viewBox="0 0 24 24"
-								stroke="currentColor"
-								stroke-width="2"
-								aria-hidden="true"
-							>
-								<path stroke-linecap="round" stroke-linejoin="round" d="M6 18L18 6M6 6l12 12" />
-							</svg>
-						</button>
 					</a>
+					<button
+						type="button"
+						data-testid="chronik-fuerdich-dismiss"
+						aria-label={m.chronik_mark_read_aria()}
+						onclick={() => onMarkRead(n)}
+						class="mt-0.5 shrink-0 rounded-sm p-1 text-ink-3 transition-colors hover:bg-muted hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:outline-none"
+					>
+						<svg
+							xmlns="http://www.w3.org/2000/svg"
+							class="h-4 w-4"
+							fill="none"
+							viewBox="0 0 24 24"
+							stroke="currentColor"
+							stroke-width="2"
+							aria-hidden="true"
+						>
+							<path stroke-linecap="round" stroke-linejoin="round" d="M6 18L18 6M6 6l12 12" />
+						</svg>
+					</button>
 				</li>
 			{/each}
 		</ul>

frontend/src/lib/components/chronik/ChronikFuerDichBox.svelte.spec.ts

@@ -113,4 +113,17 @@ describe('ChronikFuerDichBox', () => {
 		expect(onMarkRead).toHaveBeenCalledTimes(1);
 		expect(onMarkRead.mock.calls[0][0]).toEqual(n);
 	});
+
+	it('Dismiss button is a sibling of the document link, never nested inside <a>', async () => {
+		render(ChronikFuerDichBox, {
+			unread: [notif({ id: 'x' })],
+			onMarkRead: vi.fn(),
+			onMarkAllRead: vi.fn()
+		});
+		const dismiss = document.querySelector('[data-testid="chronik-fuerdich-dismiss"]');
+		expect(dismiss).not.toBeNull();
+		// HTML spec forbids interactive content descendants of <a>.
+		// Prevents the senior-audience tap-drag bug flagged by Leonie.
+		expect(dismiss?.closest('a')).toBeNull();
+	});
 });