fix: guard setState/context against disposed widget

lib/screens/auth_screen.dart

@@ -118,6 +118,7 @@ class _AuthScreenState extends State<AuthScreen> {
       Navigator.pushReplacement(context, fadeRoute(MainScreen(client: result.firstClient!)));
     } catch (e) {
       appLogger.e('Failed to connect to servers', error: e);
+      if (!mounted) return;
       setState(() {
         _isAuthenticating = false;
         _errorMessage = t.serverSelection.failedToLoadServers(error: e);

lib/screens/discover_screen.dart

@@ -570,6 +570,7 @@ class _DiscoverScreenState extends State<DiscoverScreen>
       appLogger.d('Discover content loaded successfully');
     } catch (e) {
       appLogger.e('Failed to load discover content', error: e);
+      if (!mounted) return;
       setState(() {
         _errorMessage = 'Failed to load content: $e';
         _isLoading = false;

lib/screens/libraries/libraries_screen.dart

@@ -132,6 +132,7 @@ class _LibrariesScreenState extends State<LibrariesScreen>
 
     // Initialize with libraries from the provider
     WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted) return;
       _initializeWithLibraries();
     });
   }