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/message/JoinGroupScreen.tsx

@@ -17,7 +17,7 @@ import { spacing, borderRadius, fontSizes, useAppColors, type AppColors } from '
 import Avatar from '../../components/common/Avatar';
 import Text from '../../components/common/Text';
 import { groupService } from '@/services/message';
-import { groupManager } from '../../stores/groupManager';
+import { groupManager } from '../../stores/group';
 import { GroupResponse, JoinType } from '../../types/dto';
 import { useCursorPagination } from '../../hooks/useCursorPagination';
 import { EmptyState } from '../../components/common';