fix(app): fail closed on invalid connection mode state

Restore SOCKS5 only from the current proxy value and keep UI state unchanged when persistence fails.

lib/logic/connection_settings.dart

@@ -18,23 +18,32 @@ abstract final class ConnectionSettings {
     final prefs = await SharedPreferences.getInstance();
     final storedMode = prefs.getString(_modeKey);
 
-    // Any unrecognized value (including the pre-V3 'auto') is TUN. 'proxy' is
-    // the only persisted value that selects the SOCKS5 mode.
+    // Only the explicit current proxy value restores SOCKS5. Unknown and
+    // obsolete values, including the former 'auto'/'ass' labels, fail closed
+    // to full-tunnel TUN instead of silently selecting a weaker route.
     _mode = storedMode == ConnectionMode.proxy.name
         ? ConnectionMode.proxy
         : ConnectionMode.tun;
 
     if (storedMode != _mode.name) {
-      await prefs.setString(_modeKey, _mode.name);
+      final migrated = await prefs.setString(_modeKey, _mode.name);
+      if (!migrated) {
+        throw StateError('Could not persist migrated connection mode.');
+      }
     }
 
     _initialized = true;
   }
 
-  static Future<void> setMode(ConnectionMode mode) async {
+  static Future<bool> setMode(ConnectionMode mode) async {
+    if (_initialized && _mode == mode) return true;
+
+    final prefs = await SharedPreferences.getInstance();
+    final saved = await prefs.setString(_modeKey, mode.name);
+    if (!saved) return false;
+
     _mode = mode;
     _initialized = true;
-    final prefs = await SharedPreferences.getInstance();
-    await prefs.setString(_modeKey, mode.name);
+    return true;
   }
 }