Add Last.fm integration

Related to #71 Add Last.fm integration to the app. * **lib/types/extern_api.dart**: Add `LastFmApi` class to interact with the Last.fm API, including methods for authentication and recording played songs. * **lib/audioControl/audio_controller.dart**: Add `recordToLastFm` method to handle recording played songs to Last.fm. Call `recordToLastFm` method after a song is played. * **lib/utils/global_vars.dart**: Initialize `LastFmApi` with the user's Last.fm API key and secret in the global configuration. * **lib/pages/more_page.dart**: Add a new section in the settings page for Last.fm integration. Include text fields for the user to input the Last.fm API key and secret. Save the API key and secret in the global configuration. --- For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/canxin121/app_rhyme/issues/71?shareId=dd45c9f9-4ef6-4190-b6ea-6a757e16b64e).