feat(service): add foreground service for background message sync

Implement Android foreground service to keep app alive in background for real-time message sync:
- Add withForegroundService expo config plugin for notification channel setup
- Add BackgroundSyncManager with REALTIME, BATTERY_SAVER, and DISABLED modes
- Add ForegroundServiceModule for native Android foreground service binding
- Refactor backgroundService to integrate foreground service and expo-background-task

Also refactor auth and profile screens to use dynamic theme colors:
- Replace hardcoded THEME_COLORS with colors.primary.main, colors.background.paper, etc.
- Add useResolvedColorScheme for dynamic StatusBar styling
- Update NotificationSettingsScreen with sync mode selection UI

BREAKING CHANGE: Switched from expo-background-fetch to expo-background-task, minimum background sync interval changed from 900s to 15min in config

src/screens/auth/RegisterScreen.tsx

@@ -25,7 +25,7 @@ import {
 import { SafeAreaView } from 'react-native-safe-area-context';
 import { useRouter } from 'expo-router';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
-import { useAppColors, type AppColors } from '../../theme';
+import { useAppColors, useResolvedColorScheme, type AppColors } from '../../theme';
 import { authService, resolveAuthApiError } from '../../services/authService';
 import { useAuthStore } from '../../stores';
 import {
@@ -58,6 +58,7 @@ const STEP_COMPONENTS = [
 
 export const RegisterScreen: React.FC = () => {
   const colors = useAppColors();
+  const resolved = useResolvedColorScheme();
   const styles = useMemo(() => createRegisterStyles(colors), [colors]);
   const router = useRouter();
   const register = useAuthStore((state) => state.register);
@@ -232,7 +233,7 @@ export const RegisterScreen: React.FC = () => {
 
   return (
     <SafeAreaView style={styles.container}>
-      <StatusBar barStyle="dark-content" backgroundColor="#fff" />
+      <StatusBar barStyle={resolved === 'dark' ? 'light-content' : 'dark-content'} backgroundColor={colors.background.paper} />
       <KeyboardAvoidingView
         behavior={Platform.OS === 'ios' ? 'padding' : 'height'}
         style={styles.keyboardView}
@@ -295,7 +296,7 @@ function createRegisterStyles(colors: AppColors) {
   return StyleSheet.create({
     container: {
       flex: 1,
-      backgroundColor: '#fff',
+      backgroundColor: colors.background.paper,
     },
     keyboardView: {
       flex: 1,
@@ -316,7 +317,7 @@ function createRegisterStyles(colors: AppColors) {
       width: 40,
       height: 40,
       borderRadius: 20,
-      backgroundColor: '#F5F5F7',
+      backgroundColor: colors.background.default,
       alignItems: 'center',
       justifyContent: 'center',
     },
@@ -346,7 +347,7 @@ function createRegisterStyles(colors: AppColors) {
     },
     loginLink: {
       fontSize: 15,
-      color: '#FF6B35',
+      color: colors.primary.main,
       fontWeight: '600',
       marginLeft: 6,
     },