refactor: review round 2 — remove dead code, fix type safety, improve UX

- Remove LinkOAuthCallback.tsx (dead code) and its route from App.tsx
- Move LINK_OAUTH_* constants to OAuthCallback.tsx
- Replace local ServerLinkResult with LinkCallbackResponse from types
- Move merge navigate() from render body to useEffect
- Track errorMode to show correct CTA (Return to Telegram vs Back to Login)
- Remove non-null assertions with proper narrowing
- Extract getErrorDetail helper for error casting
- Add smart polling success detection (stop + toast when linked count increases)

src/App.tsx

@@ -40,7 +40,6 @@ const ConnectionQR = lazy(() => import('./pages/ConnectionQR'));
 const TopUpMethodSelect = lazy(() => import('./pages/TopUpMethodSelect'));
 const TopUpAmount = lazy(() => import('./pages/TopUpAmount'));
 const ConnectedAccounts = lazy(() => import('./pages/ConnectedAccounts'));
-const LinkOAuthCallback = lazy(() => import('./pages/LinkOAuthCallback'));
 const LinkTelegramCallback = lazy(() => import('./pages/LinkTelegramCallback'));
 const MergeAccounts = lazy(() => import('./pages/MergeAccounts'));
 
@@ -317,16 +316,6 @@ function App() {
             </ProtectedRoute>
           }
         />
-        <Route
-          path="/auth/link/callback"
-          element={
-            <ProtectedRoute>
-              <LazyPage>
-                <LinkOAuthCallback />
-              </LazyPage>
-            </ProtectedRoute>
-          }
-        />
         <Route
           path="/auth/link/telegram/callback"
           element={