refactor: move shared utilities to lib/shared/ sub-packages

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

frontend/src/lib/document/ThumbnailRow.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 import ConversationThumbnail from '$lib/conversation/ConversationThumbnail.svelte';
 import TagChipList from '$lib/tag/TagChipList.svelte';
-import { formatDate } from '$lib/utils/date';
+import { formatDate } from '$lib/shared/utils/date';
 import * as m from '$lib/paraglide/messages.js';
 
 type Person = { id: string; firstName?: string | null; lastName: string; displayName: string };