Add MassData feature with API, paging, and Blazor grid

Introduces MassData management to backend and Blazor frontend:
- Adds API endpoint for MassData count and paging
- Updates repository and controller for count support
- Implements MediatR query/handler for count
- Adds Blazor page and grid for viewing/editing MassData
- Registers MassDataApiClient and integrates with DI
- Supports paging, upsert, and UI feedback in grid

DbFirst.BlazorWebApp/Models/MassDataReadDto.cs

@@ -0,0 +1,12 @@
+namespace DbFirst.BlazorWebApp.Models;
+
+public class MassDataReadDto
+{
+    public int Id { get; set; }
+    public string CustomerName { get; set; } = string.Empty;
+    public decimal Amount { get; set; }
+    public string Category { get; set; } = string.Empty;
+    public bool StatusFlag { get; set; }
+    public DateTime AddedWhen { get; set; }
+    public DateTime? ChangedWhen { get; set; }
+}

DbFirst.BlazorWebApp/Models/MassDataWriteDto.cs

@@ -0,0 +1,9 @@
+namespace DbFirst.BlazorWebApp.Models;
+
+public class MassDataWriteDto
+{
+    public string CustomerName { get; set; } = string.Empty;
+    public decimal Amount { get; set; }
+    public string Category { get; set; } = string.Empty;
+    public bool StatusFlag { get; set; }
+}