refactor(frontend): fix undefined --danger-color, dedup formatDate & require2FAOptions, drop double plan.css import

- base.css: var(--danger-color) (undefined, no fallback -> inherited color) -> var(--danger)
- attendanceHelpers: re-export formatDate from formatters instead of a byte-identical copy
- dashboard.ts: remove dead duplicate require2FAOptions (canonical copy is in settings.ts)
- settings.ts: remove unused @deprecated systemSettingsOptions alias
- PlanWork: drop redundant per-page plan.css import (loaded globally in AdminApp)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/lib/queries/settings.ts

@@ -76,9 +76,6 @@ export const systemInfoOptions = () =>
       ),
   });
 
-/** @deprecated Use systemInfoOptions instead — this query fetches system-info, not system settings. */
-export const systemSettingsOptions = systemInfoOptions;
-
 export const require2FAOptions = () =>
   queryOptions({
     queryKey: ["settings", "2fa"],