refactor(personFormat): replace getInitials(Person) with getInitials(name: string)

Unify the initials-extraction logic: the new string-based getInitials()
splits on whitespace, takes the first char of the first and last word
uppercased — matching the pattern that was already inlined in
CommentThread. Update PersonChip, DocumentMetadataDrawer, and
CommentThread to use the shared function.

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

frontend/src/lib/components/PersonChip.svelte

@@ -12,7 +12,7 @@ let { person, abbreviated }: Props = $props();
 
 const name = $derived(abbreviated ? abbreviateName(person) : person.displayName);
 const avatarColor = $derived(personAvatarColor(person.id));
-const initials = $derived(getInitials(person));
+const initials = $derived(getInitials(person.displayName));
 </script>
 
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