fix: resolve SDK v3 mount errors, back button and fullscreen not working

- Обернуть каждый mount-вызов в safeMountSync() для защиты от
  ConcurrentCallError при HMR/повторной инициализации
- Перенести requestFullscreen() внутрь mountViewport().then() — fullscreen
  требует смонтированный viewport, ранее вызывался через dynamic import
  с setTimeout до завершения mount
- Стабилизировать обработчик BackButton через useRef + useCallback —
  navigate меняется каждый рендер, что вызывало постоянную
  пере-подписку и потерю обработчика

src/AppWithNavigator.tsx

@@ -1,4 +1,4 @@
-import { useEffect } from 'react';
+import { useEffect, useRef, useCallback } from 'react';
 import { BrowserRouter, useLocation, useNavigate } from 'react-router-dom';
 import {
   showBackButton,
@@ -21,6 +21,8 @@ import { isInTelegramWebApp } from './hooks/useTelegramSDK';
 function TelegramBackButton() {
   const location = useLocation();
   const navigate = useNavigate();
+  const navigateRef = useRef(navigate);
+  navigateRef.current = navigate;
 
   useEffect(() => {
     const isRoot = location.pathname === '/' || location.pathname === '';
@@ -31,25 +33,29 @@ function TelegramBackButton() {
         showBackButton();
       }
     } catch {
-      // Not in Telegram or back button not mounted
+      // Back button not mounted
     }
   }, [location]);
 
+  // Stable handler — ref prevents re-subscription on every render
+  const handler = useCallback(() => {
+    navigateRef.current(-1);
+  }, []);
+
   useEffect(() => {
-    const handler = () => navigate(-1);
     try {
       onBackButtonClick(handler);
     } catch {
-      // Not in Telegram
+      // Back button not mounted
     }
     return () => {
       try {
         offBackButtonClick(handler);
       } catch {
-        // Not in Telegram
+        // Back button not mounted
       }
     };
-  }, [navigate]);
+  }, [handler]);
 
   return null;
 }

src/main.tsx

@@ -17,6 +17,8 @@ import {
   mountMainButton,
   bindThemeParamsCssVars,
   bindViewportCssVars,
+  requestFullscreen,
+  isFullscreen,
 } from '@telegram-apps/sdk-react';
 import { AppWithNavigator } from './AppWithNavigator';
 import { initLogoPreload } from './api/branding';
@@ -24,40 +26,52 @@ import { getCachedFullscreenEnabled, isTelegramMobile } from './hooks/useTelegra
 import './i18n';
 import './styles/globals.css';
 
+// Safe mount helper — ignores "already mounted/mounting" errors (HMR, StrictMode)
+function safeMountSync(fn: () => void) {
+  try {
+    fn();
+  } catch {
+    // Already mounted or not available
+  }
+}
+
 // Initialize Telegram SDK v3
 try {
   init();
   restoreInitData();
 
-  mountMiniApp();
+  safeMountSync(() => mountMiniApp());
+  safeMountSync(() => {
     mountThemeParams();
     bindThemeParamsCssVars();
+  });
+  safeMountSync(() => {
     mountSwipeBehavior();
     disableVerticalSwipes();
+  });
+  safeMountSync(() => {
     mountClosingBehavior();
     disableClosingConfirmation();
-  mountBackButton();
+  });
-  mountMainButton();
+  safeMountSync(() => mountBackButton());
+  safeMountSync(() => mountMainButton());
 
+  // Viewport — async, fullscreen зависит от смонтированного viewport
   mountViewport()
     .then(() => {
       bindViewportCssVars();
       expandViewport();
+
+      // Auto-enter fullscreen if enabled in settings (mobile only)
+      if (getCachedFullscreenEnabled() && isTelegramMobile()) {
+        if (!isFullscreen()) {
+          requestFullscreen();
+        }
+      }
     })
     .catch(() => {});
 
   miniAppReady();
-
-  // Auto-enter fullscreen if enabled in settings (mobile only)
-  if (getCachedFullscreenEnabled() && isTelegramMobile()) {
-    import('@telegram-apps/sdk-react').then(({ requestFullscreen, isFullscreen }) => {
-      setTimeout(() => {
-        if (!isFullscreen()) {
-          requestFullscreen();
-        }
-      }, 100);
-    });
-  }
 } catch {
   // Not in Telegram — ok
 }