Update dashboard navigation and dynamic loading

- Changed NavMenu to link to /dashboards instead of /dashboards/default
- Refactored Dashboard.razor to list dashboards from API
- Dashboard viewer/designer now loads by selected dashboard ID
- Mode toggle preserves selected dashboard and mode
- Added DashboardApiClient and DashboardInfoDto for API integration
- Registered DashboardApiClient for DI and HTTP client setup in Program.cs

DbFirst.BlazorWebApp/Models/DashboardInfoDto.cs

@@ -0,0 +1,7 @@
+namespace DbFirst.BlazorWebApp.Models;
+
+public class DashboardInfoDto
+{
+    public string Id { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+}