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/pages/PlanWork.tsx

@@ -15,7 +15,7 @@ import PlanCategoriesModal from "../components/PlanCategoriesModal";
 import Forbidden from "../components/Forbidden";
 import { projectListOptions, type Project } from "../lib/queries/projects";
 import { planCategoriesOptions, type PlanCategory } from "../lib/queries/plan";
-import "../plan.css";
+// plan.css is imported once globally in AdminApp.tsx — no per-page re-import.
 
 const MONTH_NAMES = [
   "Leden",