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/base.css

@@ -265,7 +265,7 @@ img {
   border-radius: var(--border-radius-sm);
   background: var(--bg-tertiary);
   border: 1px solid var(--border-color);
-  color: var(--danger-color);
+  color: var(--danger);
   font-family: var(--font-mono);
   font-size: 11px;
   line-height: 1.5;

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"],

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",

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 => {