fix(dashboard): fix ContributorStack each-block key and add accessible avatar labels

- Replace (actor.name ?? actor.initials + i) with (actor.initials + '-' + actor.color)
  to fix operator-precedence bug that made keys order-dependent when name is null
- Add role="img" + aria-label={actor.name ?? actor.initials} so screen readers
  and touch users can access contributor names

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

frontend/src/lib/components/ContributorStack.svelte.spec.ts

@@ -0,0 +1,50 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+
+import ContributorStack from './ContributorStack.svelte';
+import type { components } from '$lib/generated/api';
+
+type ActivityActorDTO = components['schemas']['ActivityActorDTO'];
+
+afterEach(() => cleanup());
+
+const makeActor = (overrides: Partial<ActivityActorDTO> = {}): ActivityActorDTO => ({
+	initials: 'MR',
+	color: '#7a4f9a',
+	name: 'Max Raddatz',
+	...overrides
+});
+
+describe('ContributorStack', () => {
+	it('contributor avatar is announced by screen readers with actor name', async () => {
+		const actor = makeActor({ name: 'Anna Meier', initials: 'AM' });
+		render(ContributorStack, { contributors: [actor], hasMore: false });
+		await expect.element(page.getByRole('img', { name: 'Anna Meier' })).toBeInTheDocument();
+	});
+
+	it('falls back to initials as accessible name when actor name is null', async () => {
+		const actor = makeActor({ name: undefined, initials: 'AM' });
+		render(ContributorStack, { contributors: [actor], hasMore: false });
+		await expect.element(page.getByRole('img', { name: 'AM' })).toBeInTheDocument();
+	});
+
+	it('renders two avatars without crashing when actors have identical initials', async () => {
+		const actors = [
+			makeActor({ name: undefined, initials: 'AM', color: '#aa0000' }),
+			makeActor({ name: undefined, initials: 'AM', color: '#0000bb' })
+		];
+		render(ContributorStack, { contributors: actors, hasMore: false });
+		await expect.element(page.getByText('AM').first()).toBeInTheDocument();
+	});
+
+	it('renders overflow indicator when hasMore is true', async () => {
+		render(ContributorStack, { contributors: [makeActor()], hasMore: true });
+		await expect.element(page.getByText('…')).toBeInTheDocument();
+	});
+
+	it('renders empty placeholder when no contributors', async () => {
+		render(ContributorStack, { contributors: [], hasMore: false });
+		await expect.element(page.getByTitle('Noch niemand angefangen')).toBeInTheDocument();
+	});
+});