feat: lazy load products in both stores (infinite scroll)

- ProductService: add GetProductsPagedAsync with PaginationState support
- Store/Products: replace load-all with page-by-page lazy loading (12 per page)
- DiscountStore/Products: replace pagination with lazy load (load more button)
- Both stores show 'load more' button + loading indicator + 'all shown' text

src/FrontOffice.Main/Pages/DiscountStore/Products.razor

@@ -64,7 +64,7 @@
             <MudText Class="mt-2 mud-text-secondary">در حال بارگذاری...</MudText>
         </MudStack>
     }
-    else if (_result.Products.Count == 0)
+    else if (_products.Count == 0)
     {
         <MudAlert Severity="Severity.Info" Class="my-4">محصولی یافت نشد.</MudAlert>
     }
@@ -72,7 +72,7 @@
     {
         <!-- Products Grid -->
         <MudGrid Spacing="1">
-            @foreach (var p in _result.Products)
+            @foreach (var p in _products)
             {
                 <MudItem xs="6" sm="6" md="3"
                          onclick="@(() => NavigateToProduct(p.Id))">
@@ -119,15 +119,29 @@
             }
         </MudGrid>
 
-        <!-- Pagination -->
-        @if (_result.TotalPages > 1)
+        @* Lazy Load — بارگذاری بیشتر *@
+        @if (_loadingMore)
         {
-            <div class="d-flex justify-center mt-4">
-                <MudPagination Count="@_result.TotalPages" Selected="@_currentPage"
-                               SelectedChanged="OnPageChanged"
-                               Color="Color.Primary" Variant="Variant.Filled"
-                               BoundaryCount="1" MiddleCount="3" />
+            <MudStack AlignItems="AlignItems.Center" Class="py-4">
+                <MudProgressCircular Color="Color.Primary" Size="Size.Small" Indeterminate="true"/>
+                <MudText Typo="Typo.caption" Class="mud-text-secondary">بارگذاری بیشتر...</MudText>
+            </MudStack>
+        }
+        else if (_hasMore)
+        {
+            <div class="d-flex justify-center py-4">
+                <MudButton Variant="Variant.Outlined" Color="Color.Primary"
+                           StartIcon="@Icons.Material.Filled.ExpandMore"
+                           OnClick="LoadMore">
+                    نمایش محصولات بیشتر
+                </MudButton>
             </div>
         }
+        else if (_products.Count > 0)
+        {
+            <MudText Typo="Typo.caption" Align="Align.Center" Class="py-4 mud-text-secondary">
+                همه @(_products.Count) محصول نمایش داده شد
+            </MudText>
+        }
     }
 </MudContainer>

src/FrontOffice.Main/Pages/DiscountStore/Products.razor.cs

@@ -13,9 +13,12 @@ public partial class Products : ComponentBase, IDisposable
     private long? _selectedCategoryId;
     private int _currentPage = 1;
     private bool _loading;
+    private bool _loadingMore;
+    private bool _hasMore = true;
+    private int _totalCount;
     private const int PageSize = 12;
 
-    private DiscountProductListResult _result = new(new(), 0, 0, 1);
+    private List<DiscountProductCard> _products = new();
     private List<DiscountCategoryNode> _categories = new();
 
     protected override async Task OnInitializedAsync()
@@ -27,45 +30,66 @@ public partial class Products : ComponentBase, IDisposable
         var productsTask = ProductService.GetProductsAsync(page: 1, pageSize: PageSize);
         await Task.WhenAll(categoriesTask, productsTask);
         _categories = categoriesTask.Result;
-        _result = productsTask.Result;
+        var result = productsTask.Result;
+        _products = result.Products;
+        _totalCount = result.TotalCount;
+        _hasMore = result.CurrentPage < result.TotalPages;
         _loading = false;
     }
 
-    private async Task LoadProductsAsync()
+    private async Task LoadInitial()
     {
         _loading = true;
+        _currentPage = 1;
+        _products.Clear();
         StateHasChanged();
-        _result = await ProductService.GetProductsAsync(
+        
+        var result = await ProductService.GetProductsAsync(
+            page: 1,
+            pageSize: PageSize,
+            search: string.IsNullOrWhiteSpace(_search) ? null : _search,
+            categoryId: _selectedCategoryId);
+        _products = result.Products;
+        _totalCount = result.TotalCount;
+        _hasMore = result.CurrentPage < result.TotalPages;
+        _loading = false;
+        StateHasChanged();
+    }
+
+    private async Task LoadMore()
+    {
+        if (_loadingMore || !_hasMore) return;
+        
+        _loadingMore = true;
+        StateHasChanged();
+        
+        _currentPage++;
+        var result = await ProductService.GetProductsAsync(
             page: _currentPage,
             pageSize: PageSize,
             search: string.IsNullOrWhiteSpace(_search) ? null : _search,
             categoryId: _selectedCategoryId);
-        _loading = false;
+        _products.AddRange(result.Products);
+        _hasMore = result.CurrentPage < result.TotalPages;
+        
+        _loadingMore = false;
         StateHasChanged();
     }
 
-    private async Task SearchProducts() => await LoadProductsAsync();
+    private async Task SearchProducts() => await LoadInitial();
 
     private async Task OnSearchKeyUp(KeyboardEventArgs e)
     {
         if (e.Key == "Enter")
         {
-            _currentPage = 1;
-            await LoadProductsAsync();
+            await LoadInitial();
         }
     }
 
     private async Task OnCategoryChanged(long? value)
     {
         _selectedCategoryId = value;
-        _currentPage = 1;
-        await LoadProductsAsync();
-    }
-
-    private async Task OnPageChanged(int page)
-    {
-        _currentPage = page;
-        await LoadProductsAsync();
+        await LoadInitial();
     }
 
     private async Task AddToCart(DiscountProductCard p)