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/home/PostDetailScreen.tsx

@@ -35,7 +35,7 @@ import {
 } from '../../theme';
 import { Post, Comment, VoteResultDTO, VoteOptionDTO } from '../../types';
 import { useUserStore } from '../../stores';
-import { useCurrentUser } from '../../stores/authStore';
+import { useCurrentUser } from '../../stores/auth';
 import { postService, commentService, uploadService, authService, showPrompt, voteService } from '../../services';
 import { postSyncService } from '@/services/post';
 import { useCursorPagination } from '../../hooks/useCursorPagination';