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/navigation/hrefs.ts

@@ -2,7 +2,7 @@
  * Expo Router 路径构建（单一事实来源，避免魔法字符串散落）
  */
 import type { SystemMessageResponse } from '../types/dto';
-import { routePayloadCache } from '../stores/routePayloadCache';
+import { routePayloadCache } from '../stores';
 
 export function hrefPostDetail(postId: string, scrollToComments?: boolean): string {
   const q = scrollToComments ? '?scrollToComments=1' : '';