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/utils/attendanceHelpers.ts

@@ -21,11 +21,9 @@ interface AttendanceRecord {
   }>;
 }
 
-export const formatDate = (dateStr: string | null | undefined): string => {
-  if (!dateStr) return "—";
-  const d = new Date(dateStr);
-  return d.toLocaleDateString("cs-CZ");
-};
+// formatDate is defined once in ./formatters and re-exported here so existing
+// importers of attendanceHelpers keep working without a duplicate definition.
+export { formatDate } from "./formatters";
 
 /** Extract time as HH:MM from a datetime string without timezone conversion */
 const extractTime = (datetime: string): string => {