test(chronik): extract applyClientFilter helper with full test coverage

Addresses review concern: the fuer-dich predicate (youMentioned ||
youParticipated) had zero test coverage after feedFilters.test.ts was
deleted. The new clientFilter module is a pure function that is directly
testable, and the test explicitly documents why MENTION_CREATED items
without the youMentioned flag are now excluded (they would have shown
mentions directed at OTHER users under the old feedFilters.ts logic).

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

frontend/src/routes/chronik/+page.svelte

@@ -11,6 +11,7 @@ import ChronikEmptyState from '$lib/components/chronik/ChronikEmptyState.svelte'
 import ChronikErrorCard from '$lib/components/chronik/ChronikErrorCard.svelte';
 import type { components } from '$lib/generated/api';
 import type { FilterValue } from './+page.server';
+import { applyClientFilter } from './clientFilter';
 
 type ActivityFeedItemDTO = components['schemas']['ActivityFeedItemDTO'];
 
@@ -83,13 +84,7 @@ async function onMarkAllRead() {
 	await notificationStore.markAllRead();
 }
 
-// fuer-dich cannot be expressed as a server-side kinds filter — youMentioned/youParticipated
-// are user-scoped flags that require client-side narrowing
-const displayFeed = $derived(
-	data.filter === 'fuer-dich'
-		? data.activityFeed.filter((item) => item.youMentioned || item.youParticipated)
-		: data.activityFeed
-);
+const displayFeed = $derived(applyClientFilter(data.activityFeed, data.filter));
 
 const isEmpty = $derived(displayFeed.length === 0);
 const emptyVariant = $derived<'first-run' | 'filter-empty' | 'inbox-zero'>(

frontend/src/routes/chronik/clientFilter.test.ts

@@ -0,0 +1,94 @@
+import { describe, expect, it } from 'vitest';
+import { applyClientFilter } from './clientFilter';
+import type { components } from '$lib/generated/api';
+
+type Item = components['schemas']['ActivityFeedItemDTO'];
+
+function makeItem(overrides: Partial<Item> = {}): Item {
+	return {
+		kind: 'FILE_UPLOADED',
+		documentId: 'd1',
+		documentTitle: 'Brief A',
+		happenedAt: '2026-04-20T10:00:00Z',
+		youMentioned: false,
+		youParticipated: false,
+		count: 1,
+		actor: null,
+		happenedAtUntil: null,
+		...overrides
+	};
+}
+
+describe('applyClientFilter', () => {
+	describe('non-fuer-dich filters pass through unchanged', () => {
+		it('alle returns all items', () => {
+			const items = [makeItem(), makeItem({ kind: 'COMMENT_ADDED' })];
+			expect(applyClientFilter(items, 'alle')).toHaveLength(2);
+		});
+
+		it('hochgeladen passes through (server already filtered by kinds)', () => {
+			const items = [makeItem({ kind: 'FILE_UPLOADED' }), makeItem({ kind: 'COMMENT_ADDED' })];
+			expect(applyClientFilter(items, 'hochgeladen')).toHaveLength(2);
+		});
+
+		it('transkription passes through (server already filtered by kinds)', () => {
+			const items = [makeItem({ kind: 'TEXT_SAVED' })];
+			expect(applyClientFilter(items, 'transkription')).toHaveLength(1);
+		});
+
+		it('kommentare passes through (server already filtered by kinds)', () => {
+			const items = [makeItem({ kind: 'COMMENT_ADDED' })];
+			expect(applyClientFilter(items, 'kommentare')).toHaveLength(1);
+		});
+	});
+
+	describe('fuer-dich applies youMentioned || youParticipated predicate', () => {
+		it('includes items where youMentioned is true', () => {
+			const items = [makeItem({ youMentioned: true })];
+			expect(applyClientFilter(items, 'fuer-dich')).toHaveLength(1);
+		});
+
+		it('includes items where youParticipated is true', () => {
+			const items = [makeItem({ youParticipated: true })];
+			expect(applyClientFilter(items, 'fuer-dich')).toHaveLength(1);
+		});
+
+		it('includes items where both flags are true', () => {
+			const items = [makeItem({ youMentioned: true, youParticipated: true })];
+			expect(applyClientFilter(items, 'fuer-dich')).toHaveLength(1);
+		});
+
+		it('excludes items where neither flag is set', () => {
+			const items = [makeItem({ kind: 'COMMENT_ADDED' })];
+			expect(applyClientFilter(items, 'fuer-dich')).toHaveLength(0);
+		});
+
+		it('MENTION_CREATED without youMentioned flag is excluded', () => {
+			// youMentioned is set by the backend only when this event is directed at the current user.
+			// Unlike the old feedFilters.ts, we no longer include MENTION_CREATED unconditionally —
+			// that incorrectly showed mentions directed at OTHER users.
+			const items = [
+				makeItem({ kind: 'MENTION_CREATED', youMentioned: false, youParticipated: false })
+			];
+			expect(applyClientFilter(items, 'fuer-dich')).toHaveLength(0);
+		});
+
+		it('MENTION_CREATED with youMentioned true is included', () => {
+			const items = [makeItem({ kind: 'MENTION_CREATED', youMentioned: true })];
+			expect(applyClientFilter(items, 'fuer-dich')).toHaveLength(1);
+		});
+
+		it('filters mixed items to only personally relevant ones', () => {
+			const items = [
+				makeItem({ kind: 'FILE_UPLOADED' }),
+				makeItem({ kind: 'COMMENT_ADDED', youParticipated: true }),
+				makeItem({ kind: 'MENTION_CREATED', youMentioned: true }),
+				makeItem({ kind: 'TEXT_SAVED' })
+			];
+			const result = applyClientFilter(items, 'fuer-dich');
+			expect(result).toHaveLength(2);
+			expect(result[0].kind).toBe('COMMENT_ADDED');
+			expect(result[1].kind).toBe('MENTION_CREATED');
+		});
+	});
+});

frontend/src/routes/chronik/clientFilter.ts

@@ -0,0 +1,18 @@
+import type { components } from '$lib/generated/api';
+import type { FilterValue } from './+page.server';
+
+type ActivityFeedItemDTO = components['schemas']['ActivityFeedItemDTO'];
+
+// All server-side filters (hochgeladen, transkription, kommentare) are already applied via
+// kinds param — the server returns only matching items, so client-side is a no-op.
+// fuer-dich is the only filter that requires client-side narrowing: youMentioned and
+// youParticipated are user-scoped flags that cannot be expressed as a kinds filter.
+export function applyClientFilter(
+	items: ActivityFeedItemDTO[],
+	filter: FilterValue
+): ActivityFeedItemDTO[] {
+	if (filter === 'fuer-dich') {
+		return items.filter((item) => item.youMentioned || item.youParticipated);
+	}
+	return items;
+}