refactor(date): consolidate formatDate in date.ts with optional format param

Add format?: 'short'|'long' (default 'long') to date.ts formatDate and
remove the duplicate from personFormat.ts. Update DocumentTopBar to
import from date.ts directly. Move the formatDate tests from
personFormat.spec to date.spec.

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

frontend/src/lib/utils/personFormat.spec.ts

@@ -4,10 +4,10 @@ import {
 	abbreviateName,
 	formatXsMeta,
 	personAvatarColor,
-	formatDate,
 	statusDotClass,
 	statusLabel
 } from './personFormat';
+import { formatDate } from './date';
 
 // ─── getInitials ─────────────────────────────────────────────────────────────