fix: windows single instance mutex

windows/runner/main.cpp

@@ -7,6 +7,18 @@
 
 int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
                       _In_ wchar_t *command_line, _In_ int show_command) {
+  // Single instance enforcement
+  HANDLE mutex = CreateMutex(nullptr, TRUE, L"com.edde746.Plezy.SingleInstance");
+  if (GetLastError() == ERROR_ALREADY_EXISTS) {
+    HWND existing = FindWindow(L"FLUTTER_RUNNER_WIN32_WINDOW", L"Plezy");
+    if (existing) {
+      ShowWindow(existing, SW_RESTORE);
+      SetForegroundWindow(existing);
+    }
+    CloseHandle(mutex);
+    return EXIT_SUCCESS;
+  }
+
   // Attach to console when present (e.g., 'flutter run') or create a
   // new console when running with a debugger.
   if (!::AttachConsole(ATTACH_PARENT_PROCESS) && ::IsDebuggerPresent()) {