Add configuration validation

[wizard425]

the popup happily fires GenerateFromTextAndImageAsync even when ApiKey/Model are blank (ActionlyPlugin/src/Actions/PopUp.xaml.cs (lines 63-89)). The Gemini client immediately constructs new Client(apiKey: apiKey) without checking for null/empty, which throws and collapses the workflow instead of giving the user a friendly error and staying in the dialog (ActionlyPlugin/src/Helpers/GeminiClient.cs (lines 27-78)).