feat: account linking and merge UI for cabinet

Add Connected Accounts page (link/unlink OAuth providers), Link OAuth
Callback handler, and Merge Accounts page with subscription comparison
and user choice. Includes API methods, TypeScript types, routes in
App.tsx, navigation from Profile, and i18n for all 4 locales (ru, en,
zh, fa). Merge page works without JWT auth (validated by merge token).

src/App.tsx

@@ -39,6 +39,9 @@ const Connection = lazy(() => import('./pages/Connection'));
 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 MergeAccounts = lazy(() => import('./pages/MergeAccounts'));
 
 // Admin pages - lazy load (only for admins)
 const AdminPanel = lazy(() => import('./pages/AdminPanel'));
@@ -183,6 +186,14 @@ function App() {
         <Route path="/auth/oauth/callback" element={<OAuthCallback />} />
         <Route path="/verify-email" element={<VerifyEmail />} />
         <Route path="/reset-password" element={<ResetPassword />} />
+        <Route
+          path="/merge/:mergeToken"
+          element={
+            <LazyPage>
+              <MergeAccounts />
+            </LazyPage>
+          }
+        />
 
         {/* Protected routes */}
         <Route
@@ -295,6 +306,26 @@ function App() {
             </ProtectedRoute>
           }
         />
+        <Route
+          path="/profile/accounts"
+          element={
+            <ProtectedRoute>
+              <LazyPage>
+                <ConnectedAccounts />
+              </LazyPage>
+            </ProtectedRoute>
+          }
+        />
+        <Route
+          path="/auth/link/callback"
+          element={
+            <ProtectedRoute>
+              <LazyPage>
+                <LinkOAuthCallback />
+              </LazyPage>
+            </ProtectedRoute>
+          }
+        />
         <Route
           path="/contests"
           element={