feat: permisos automáticos + cableado crear partida → lobby multi

- ServicioPermisos: solicita BT+Location automáticamente con diálogo si denegados
- PantallaCrearPartida: modo multi → pide nombre host → permisos → lobby con QR
- PantallaUnirse: pide permisos antes de iniciar discovery
- ServicioNearby: pararBusqueda() para limpiar discovery sin desconectar
- Botón iniciar habilitado en modo multi sin necesidad de 3 jugadores locales
- permission_handler añadido como dependencia

pubspec.lock

@@ -317,6 +317,54 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.3.0"
+  permission_handler:
+    dependency: "direct main"
+    description:
+      name: permission_handler
+      sha256: bc917da36261b00137bbc8896bf1482169cd76f866282368948f032c8c1caae1
+      url: "https://pub.dev"
+    source: hosted
+    version: "12.0.1"
+  permission_handler_android:
+    dependency: transitive
+    description:
+      name: permission_handler_android
+      sha256: "1e3bc410ca1bf84662104b100eb126e066cb55791b7451307f9708d4007350e6"
+      url: "https://pub.dev"
+    source: hosted
+    version: "13.0.1"
+  permission_handler_apple:
+    dependency: transitive
+    description:
+      name: permission_handler_apple
+      sha256: f000131e755c54cf4d84a5d8bd6e4149e262cc31c5a8b1d698de1ac85fa41023
+      url: "https://pub.dev"
+    source: hosted
+    version: "9.4.7"
+  permission_handler_html:
+    dependency: transitive
+    description:
+      name: permission_handler_html
+      sha256: "38f000e83355abb3392140f6bc3030660cfaef189e1f87824facb76300b4ff24"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.1.3+5"
+  permission_handler_platform_interface:
+    dependency: transitive
+    description:
+      name: permission_handler_platform_interface
+      sha256: eb99b295153abce5d683cac8c02e22faab63e50679b937fa1bf67d58bb282878
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.3.0"
+  permission_handler_windows:
+    dependency: transitive
+    description:
+      name: permission_handler_windows
+      sha256: "1a790728016f79a41216d88672dbc5df30e686e811ad4e698bfc51f76ad91f1e"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.2.1"
   platform:
     dependency: transitive
     description: