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/hooks/useSettings.ts

@@ -69,7 +69,6 @@ export function useSettings(): UseSettingsResult {
     initialLanguage,
     updateSettings,
     resetSettings: resetForm,
-    readPersistedLanguage,
     syncLanguage,
   } = useSettingsForm();