fix(web): blank-screen hardening — stale chunk recovery, error boundary, no-cache index.html

- main.tsx: on vite:preloadError (stale index.html referencing removed
  hashed chunks after a redeploy) reload the page immediately.
- New ErrorBoundary around the whole router: render errors show a Persian
  fallback with «تلاش مجدد» instead of wiping the DOM.
- nginx: index.html served with Cache-Control no-cache,no-store,
  must-revalidate (verified live); /assets/* stays public, immutable.

frontend/src/App.tsx

@@ -2,6 +2,7 @@ import { lazy, Suspense } from "react";
 import { Navigate, Route, Routes } from "react-router-dom";
 import { useAuthStore } from "./store/auth";
 import { MobileTabBar } from "./components/layout/MobileTabBar";
+import { ErrorBoundary } from "./components/ErrorBoundary";
 
 // هر صفحه با React.lazy لود می‌شود (code splitting)
 const DashboardPage = lazy(() =>
@@ -66,7 +67,7 @@ function HomeRoute() {
 
 export default function App() {
   return (
-    <>
+    <ErrorBoundary>
       <Suspense fallback={<Loading />}>
         <Routes>
         <Route path="/login" element={<LoginPage />} />
@@ -197,6 +198,6 @@ export default function App() {
         </Routes>
       </Suspense>
       <MobileTabBar />
-    </>
+    </ErrorBoundary>
   );
 }