feat(mobile): implement real Supabase authentication

Replace mock authentication with real Supabase integration:

**New Files:**
- mobile/src/lib/supabase.ts - Supabase client initialization with AsyncStorage persistence
- mobile/src/contexts/AuthContext.tsx - Complete authentication context with session management

**Updated Files:**
- mobile/src/screens/SignInScreen.tsx
  * Import useAuth from AuthContext
  * Add Alert and ActivityIndicator for error handling and loading states
  * Replace mock setTimeout with real signIn() API call
  * Add loading state management (isLoading)
  * Update button to show ActivityIndicator during sign-in
  * Add proper error handling with Alert dialogs

- mobile/src/screens/SignUpScreen.tsx
  * Import useAuth from AuthContext
  * Add Alert and ActivityIndicator
  * Add username state and input field
  * Replace mock registration with real signUp() API call
  * Add loading state management
  * Update button to show ActivityIndicator during sign-up
  * Add form validation for all required fields
  * Add proper error handling with Alert dialogs

- mobile/App.tsx
  * Import and add AuthProvider to provider hierarchy
  * Provider order: ErrorBoundary → AuthProvider → PolkadotProvider → LanguageProvider → BiometricAuthProvider

**Features Implemented:**
- Real user authentication with Supabase
- Email/password sign in with error handling
- User registration with username and referral code support
- Profile creation in Supabase database
- Admin status checking
- Session timeout management (30 minutes inactivity)
- Automatic session refresh
- Activity tracking with AsyncStorage
- Auth state persistence across app restarts

**Security:**
- Credentials from environment variables (EXPO_PUBLIC_SUPABASE_URL, EXPO_PUBLIC_SUPABASE_ANON_KEY)
- Automatic token refresh enabled
- Secure session persistence with AsyncStorage
- No sensitive data in console logs (protected with __DEV__)

This completes P0 authentication implementation for mobile app.
Production ready authentication matching web implementation.

mobile/src/lib/supabase.ts

@@ -0,0 +1,23 @@
+import 'react-native-url-polyfill/auto';
+import { createClient } from '@supabase/supabase-js';
+import AsyncStorage from '@react-native-async-storage/async-storage';
+
+// Initialize Supabase client from environment variables
+const supabaseUrl = process.env.EXPO_PUBLIC_SUPABASE_URL || '';
+const supabaseKey = process.env.EXPO_PUBLIC_SUPABASE_ANON_KEY || '';
+
+if (!supabaseUrl || !supabaseKey) {
+  if (__DEV__) {
+    console.warn('Supabase credentials not found in environment variables');
+    console.warn('Add EXPO_PUBLIC_SUPABASE_URL and EXPO_PUBLIC_SUPABASE_ANON_KEY to .env');
+  }
+}
+
+export const supabase = createClient(supabaseUrl, supabaseKey, {
+  auth: {
+    storage: AsyncStorage,
+    autoRefreshToken: true,
+    persistSession: true,
+    detectSessionInUrl: false,
+  },
+});