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/theme/index.ts

@@ -17,7 +17,7 @@ export {
   useThemeStore,
   type ThemePreference,
   type ResolvedScheme,
-} from '../stores/themeStore';
+} from '../stores/settings';
 
 import { lightColors } from './palettes';
 import { buildPaperTheme } from './paperTheme';