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/dashboard.ts

@@ -9,12 +9,8 @@ export const dashboardOptions = () =>
     staleTime: 60_000,
   });
 
-export const require2FAOptions = () =>
-  queryOptions({
-    queryKey: ["settings", "2fa"],
-    queryFn: () =>
-      jsonQuery<{ require_2fa: boolean }>("/api/admin/totp/required"),
-  });
+// require2FAOptions lives in ./settings.ts (the single definition consumers
+// import). The duplicate copy that used to be here was dead.
 
 export interface Session {
   id: number | string;

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"],