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';

src/screens/auth/VerificationFormScreen.tsx

@@ -21,7 +21,7 @@ import { useRouter, useLocalSearchParams } from 'expo-router';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
 import * as ImagePicker from 'expo-image-picker';
 import { useAppColors, type AppColors } from '../../theme';
-import { useVerificationStore } from '../../stores/verificationStore';
+import { useVerificationStore } from '../../stores/auth';
 import { showPrompt } from '@/services/ui';
 import { uploadService } from '@/services/upload';
 import type { UserIdentity } from '../../types/dto';