refactor: Phase 7-3 Complete - All Blazor pages API-First migration

**Blazor Pages Refactored (9 pages):**
✅ ClientList.razor (Service → IClientBrowserClient)
✅ ClientEdit.razor (Service → IClientBrowserClient)
✅ TaxFilingList.razor (Service → ITaxFilingBrowserClient)
✅ FilingTable.razor (Service → ITaxFilingBrowserClient)
✅ FaqList.razor (Service → IFaqBrowserClient)
✅ FaqEdit.razor (Service → IFaqBrowserClient)
✅ AnnouncementList.razor (Service → IAnnouncementBrowserClient)
✅ AnnouncementEdit.razor (Service → IAnnouncementBrowserClient)
✅ Previously: Dashboard, InquiryTable, InquiryDetail

**Pattern Applied Consistently:**
- Removed all direct service injections (Service Layer)
- Injected specialized Browser Clients (API Layer)
- Error handling with Snackbar notifications
- Try-catch for all API calls
- Graceful fallbacks (empty lists on error)

**Phase 7 Complete: 100% API-First Refactoring**

All admin pages now use:
ClientBrowserClient → /api/client (Clients)
TaxFilingBrowserClient → /api/tax-filing (Tax Filings)
FaqBrowserClient → /api/faq (FAQs)
AnnouncementBrowserClient → /api/announcement (Announcements)
InquiryBrowserClient → /api/inquiry (Inquiries)
AdminDashboardClient → /api/admin-dashboard (Dashboard)

**SOLID + Maintainability Achieved:**
✓ Single Responsibility: Each client = one domain
✓ Open/Closed: Extensible without modifying Blazor
✓ Dependency Inversion: Blazor → Abstractions, not services
✓ Interface Segregation: Fine-grained client interfaces
✓ Liskov Substitution: Interchangeable implementations

Build: ✅ Success (0 errors)
Status: Ready for Phase 6 (SignalR Integration)

Next: NotificationHub for real-time dashboard updates

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TaxBaik.Web/Components/Admin/Pages/Announcements/AnnouncementEdit.razor

@@ -2,8 +2,8 @@
 @page "/admin/announcements/{Id:int}/edit"
 @attribute [Authorize]
 @using TaxBaik.Application.DTOs
-@using TaxBaik.Application.Services
-@inject AnnouncementService AnnouncementService
+@using TaxBaik.Web.Services
+@inject IAnnouncementBrowserClient AnnouncementClient
 @inject NavigationManager Navigation
 @inject ISnackbar Snackbar
 
@@ -105,23 +105,30 @@
     {
         if (Id.HasValue)
         {
-            var entity = await AnnouncementService.GetByIdAsync(Id.Value);
-            if (entity is null)
+            try
+            {
+                var entity = await AnnouncementClient.GetByIdAsync(Id.Value);
+                if (entity is null)
+                {
+                    Navigation.NavigateTo("/taxbaik/admin/announcements");
+                    return;
+                }
+                model = new AnnouncementDto
+                {
+                    Id          = entity.Id,
+                    Title       = entity.Title,
+                    Content     = entity.Content,
+                    DisplayType = entity.DisplayType,
+                    IsActive    = entity.IsActive,
+                    SortOrder   = entity.SortOrder
+                };
+                startsAtDate = entity.StartsAt?.ToLocalTime();
+                endsAtDate   = entity.EndsAt?.ToLocalTime();
+            }
+            catch
             {
                 Navigation.NavigateTo("/taxbaik/admin/announcements");
-                return;
             }
-            model = new AnnouncementDto
-            {
-                Id          = entity.Id,
-                Title       = entity.Title,
-                Content     = entity.Content,
-                DisplayType = entity.DisplayType,
-                IsActive    = entity.IsActive,
-                SortOrder   = entity.SortOrder
-            };
-            startsAtDate = entity.StartsAt?.ToLocalTime();
-            endsAtDate   = entity.EndsAt?.ToLocalTime();
         }
     }
 
@@ -142,11 +149,21 @@
                 : null;
 
             if (Id.HasValue)
-                await AnnouncementService.UpdateAsync(model);
+            {
+                var result = await AnnouncementClient.UpdateAsync(Id.Value, model);
+                if (result != null)
+                    Snackbar.Add("공지사항이 저장되었습니다.", Severity.Success);
+                else
+                    Snackbar.Add("저장 실패", Severity.Error);
+            }
             else
-                await AnnouncementService.CreateAsync(model);
-
-            Snackbar.Add("공지사항이 저장되었습니다.", Severity.Success);
+            {
+                var result = await AnnouncementClient.CreateAsync(model);
+                if (result != null)
+                    Snackbar.Add("공지사항이 저장되었습니다.", Severity.Success);
+                else
+                    Snackbar.Add("저장 실패", Severity.Error);
+            }
             Navigation.NavigateTo("/taxbaik/admin/announcements");
         }
         catch (Exception ex)