feat: Add Instance Details ViewModel and UI for managing instance credentials

- Introduced InstanceDetailsViewModel to handle loading and displaying instance-specific credentials.
- Created InstanceDetailsWindow and associated XAML for displaying admin, database, and OAuth2 credentials.
- Updated InstancesViewModel to include command for opening instance details.
- Enhanced SettingsViewModel to manage Bitwarden and Xibo Bootstrap configurations, including connection testing.
- Added UI components for Bitwarden Secrets Manager and Xibo Bootstrap OAuth2 settings in the SettingsView.
- Implemented password visibility toggles and clipboard copy functionality for sensitive information.

OTSSignsOrchestrator.Core/Services/IBitwardenSecretService.cs

@@ -0,0 +1,49 @@
+namespace OTSSignsOrchestrator.Core.Services;
+
+/// <summary>
+/// Abstraction for storing and retrieving secrets via Bitwarden Secrets Manager.
+/// </summary>
+public interface IBitwardenSecretService
+{
+    /// <summary>
+    /// Returns true if Bitwarden is configured (access token + org ID are set).
+    /// </summary>
+    Task<bool> IsConfiguredAsync();
+
+    /// <summary>
+    /// Creates a new secret in the configured Bitwarden project.
+    /// </summary>
+    /// <returns>The ID of the created secret.</returns>
+    Task<string> CreateSecretAsync(string key, string value, string note = "");
+
+    /// <summary>
+    /// Retrieves a secret by its Bitwarden ID.
+    /// </summary>
+    Task<BitwardenSecret> GetSecretAsync(string secretId);
+
+    /// <summary>
+    /// Updates the value of an existing secret in place.
+    /// </summary>
+    Task UpdateSecretAsync(string secretId, string key, string value, string note = "");
+
+    /// <summary>
+    /// Lists all secrets in the configured project.
+    /// </summary>
+    Task<List<BitwardenSecretSummary>> ListSecretsAsync();
+}
+
+public class BitwardenSecret
+{
+    public string Id { get; set; } = string.Empty;
+    public string Key { get; set; } = string.Empty;
+    public string Value { get; set; } = string.Empty;
+    public string Note { get; set; } = string.Empty;
+    public DateTime CreationDate { get; set; }
+}
+
+public class BitwardenSecretSummary
+{
+    public string Id { get; set; } = string.Empty;
+    public string Key { get; set; } = string.Empty;
+    public DateTime CreationDate { get; set; }
+}