feat: add OAuth 2.0 login UI (Google, Yandex, Discord, VK)

- Add OAuthProvider type and extend User.auth_type union
- Add OAuth API methods (providers, authorize, callback)
- Add loginWithOAuth to auth store
- Create OAuthCallback page with state validation
- Create OAuthProviderIcon component with brand SVGs
- Add OAuth buttons to Login page between Telegram and Email
- Add OAuth callback route to App.tsx
- Add translations for ru, en, zh, fa

src/store/auth.ts

@@ -33,6 +33,7 @@ interface AuthState {
   loginWithTelegram: (initData: string) => Promise<void>;
   loginWithTelegramWidget: (data: TelegramWidgetData) => Promise<void>;
   loginWithEmail: (email: string, password: string) => Promise<void>;
+  loginWithOAuth: (provider: string, code: string, state: string) => Promise<void>;
   registerWithEmail: (
     email: string,
     password: string,
@@ -267,6 +268,18 @@ export const useAuthStore = create<AuthState>()(
         await get().checkAdminStatus();
       },
 
+      loginWithOAuth: async (provider, code, state) => {
+        const response = await authApi.oauthCallback(provider, code, state);
+        tokenStorage.setTokens(response.access_token, response.refresh_token);
+        set({
+          accessToken: response.access_token,
+          refreshToken: response.refresh_token,
+          user: response.user,
+          isAuthenticated: true,
+        });
+        await get().checkAdminStatus();
+      },
+
       registerWithEmail: async (email, password, firstName, referralCode) => {
         // Registration now returns message, not tokens
         // User must verify email before they can login