refactor: improve code quality and consistency

Changes:
1. Remove unused variable in useSettings.ts (readPersistedLanguage)
2. Replace manual state management with React Query in UsageFooter
   - Create useUsageQuery hook with 5-minute cache
   - Simplify component from 227 lines to 81 lines (-64%)
   - Improve consistency with project's React Query pattern
   - Enable automatic refetch and error handling

src/lib/query/queries.ts

@@ -1,6 +1,6 @@
 import { useQuery, type UseQueryResult } from "@tanstack/react-query";
-import { providersApi, settingsApi, type AppType } from "@/lib/api";
-import type { Provider, Settings } from "@/types";
+import { providersApi, settingsApi, usageApi, type AppType } from "@/lib/api";
+import type { Provider, Settings, UsageResult } from "@/types";
 
 const sortProviders = (
   providers: Record<string, Provider>,
@@ -77,3 +77,17 @@ export const useSettingsQuery = (): UseQueryResult<Settings> => {
     queryFn: async () => settingsApi.get(),
   });
 };
+
+export const useUsageQuery = (
+  providerId: string,
+  appType: AppType,
+  enabled: boolean = true,
+): UseQueryResult<UsageResult> => {
+  return useQuery({
+    queryKey: ["usage", providerId, appType],
+    queryFn: async () => usageApi.query(providerId, appType),
+    enabled: enabled && !!providerId,
+    refetchOnWindowFocus: false,
+    staleTime: 5 * 60 * 1000, // 5分钟
+  });
+};