refactor(stores): reorganize flat store files into modular directory structure

Migrate from flat file organization to modular directory structure under src/stores/:

- auth/: authStore, registerStore, verificationStore, sessionStore
- call/: callStore
- settings/: chatSettingsStore, themeStore
- ui/: homeTabBarVisibilityStore, homeTabPressStore
- utils/: routePayloadCache
- group/sources.ts, group/profileResolver.ts
- message/sources.ts
- post/sources.ts

Update all import paths across components and screens to use new module paths. Maintain backward compatibility through deprecated re-export files for gradual migration.

src/screens/auth/RegisterScreen.tsx

@@ -33,7 +33,7 @@ import {
   useRegisterStep,
   useRegisterFormData,
   useRegisterCountdown,
-} from '../../stores/registerStore';
+} from '../../stores/auth';
 import * as hrefs from '../../navigation/hrefs';
 import { showPrompt } from '@/services/ui';
 import { RegisterStepIndicator } from '../../components/common';