refactor(timestamp): migrate pageTheme constants to Timestamp feature

Remove config/pageTheme.ts and move DATE_FORMAT, ZONES, UnitType, ZoneType
to pages/Timestamp/constants.ts for better feature cohesion.

pages/Timestamp/LiveClock.tsx

@@ -2,7 +2,7 @@ import React, { useCallback, useEffect, useRef, useState } from 'react';
 import { Clock } from 'lucide-react';
 import CopyButton from '@/components/CopyButton';
 import { useSnackbar } from '@/components/GlobalSnackbar';
-import type { UnitType } from '@/config/pageTheme';
+import type { UnitType } from './constants';
 import { useLazyTranslation } from '@/utils/useLazyTranslation';
 import { cn } from '@/lib/utils'; // 引入标准的 shadcn 工具函数