refactor: Phase 7-3 - Clients + TaxFilings API-First (WIP)

**Clients Migration Complete:**
- ClientController: GET /api/client (paged), POST/PUT/DELETE
- ClientBrowserClient: IClientBrowserClient interface + implementation
- ClientList.razor: Service → API client
- ClientEdit.razor: Service → API client (Create/Update)

**TaxFilings API Framework Ready:**
- TaxFilingController: GET upcoming, GET by client, POST/PUT/DELETE
- TaxFilingBrowserClient: ITaxFilingBrowserClient interface + impl
- Registered in Program.cs with TokenRefreshHandler

**SOLID Applied:**
✓ Separation of concerns (Controller → Service → Repository)
✓ Dependency inversion (Blazor → Browser clients, not services)
✓ Interface segregation (Specialized clients per domain)

**Status:**
- Clients Blazor: ✅ ClientList + ClientEdit refactored
- TaxFilings Blazor: ⏳ Pending refactor (pages exist)
- Faqs: ⏳ API + Blazor pending
- Announcements: ⏳ API + Blazor pending
- Phase 6 SignalR: ⏳ Deferred

Next: Refactor TaxFilings Blazor pages, then Faqs & Announcements
Build: ✅ Success (0 errors, 2 warnings in Dashboard)

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

TaxBaik.Web/Services/TaxFilingBrowserClient.cs

@@ -0,0 +1,132 @@
+namespace TaxBaik.Web.Services;
+
+using System.Net.Http.Json;
+using TaxBaik.Domain.Entities;
+
+/// <summary>
+/// TaxFiling API Client for Admin Blazor
+/// </summary>
+public interface ITaxFilingBrowserClient
+{
+    Task<IEnumerable<TaxFiling>> GetUpcomingAsync(int daysAhead = 30, CancellationToken ct = default);
+    Task<IEnumerable<TaxFiling>> GetByClientIdAsync(int clientId, CancellationToken ct = default);
+    Task<TaxFiling?> GetByIdAsync(int id, CancellationToken ct = default);
+    Task<TaxFiling?> CreateAsync(TaxFiling filing, CancellationToken ct = default);
+    Task<TaxFiling?> UpdateAsync(int id, TaxFiling filing, CancellationToken ct = default);
+    Task<bool> DeleteAsync(int id, CancellationToken ct = default);
+}
+
+public class TaxFilingBrowserClient : ITaxFilingBrowserClient
+{
+    private readonly HttpClient _http;
+    private readonly ILogger<TaxFilingBrowserClient> _logger;
+
+    public TaxFilingBrowserClient(HttpClient http, ILogger<TaxFilingBrowserClient> logger)
+    {
+        _http = http;
+        _logger = logger;
+    }
+
+    public async Task<IEnumerable<TaxFiling>> GetUpcomingAsync(int daysAhead = 30, CancellationToken ct = default)
+    {
+        try
+        {
+            var result = await _http.GetFromJsonAsync<TaxFilingListResponse>(
+                $"tax-filing/upcoming?daysAhead={daysAhead}", cancellationToken: ct);
+            return result?.Data ?? [];
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogError(ex, "Failed to fetch upcoming filings");
+            throw;
+        }
+    }
+
+    public async Task<IEnumerable<TaxFiling>> GetByClientIdAsync(int clientId, CancellationToken ct = default)
+    {
+        try
+        {
+            var result = await _http.GetFromJsonAsync<TaxFilingListResponse>(
+                $"tax-filing/client/{clientId}", cancellationToken: ct);
+            return result?.Data ?? [];
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogError(ex, "Failed to fetch filings for client {ClientId}", clientId);
+            throw;
+        }
+    }
+
+    public async Task<TaxFiling?> GetByIdAsync(int id, CancellationToken ct = default)
+    {
+        try
+        {
+            return await _http.GetFromJsonAsync<TaxFiling>(
+                $"tax-filing/{id}", cancellationToken: ct);
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogError(ex, "Failed to fetch filing {FilingId}", id);
+            throw;
+        }
+    }
+
+    public async Task<TaxFiling?> CreateAsync(TaxFiling filing, CancellationToken ct = default)
+    {
+        try
+        {
+            var response = await _http.PostAsJsonAsync("tax-filing", filing, cancellationToken: ct);
+            if (!response.IsSuccessStatusCode)
+                return null;
+
+            var content = await response.Content.ReadAsStringAsync(ct);
+            return System.Text.Json.JsonSerializer.Deserialize<TaxFiling>(
+                content,
+                new System.Text.Json.JsonSerializerOptions { PropertyNameCaseInsensitive = true });
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogError(ex, "Failed to create filing");
+            throw;
+        }
+    }
+
+    public async Task<TaxFiling?> UpdateAsync(int id, TaxFiling filing, CancellationToken ct = default)
+    {
+        try
+        {
+            var response = await _http.PutAsJsonAsync($"tax-filing/{id}", filing, cancellationToken: ct);
+            if (!response.IsSuccessStatusCode)
+                return null;
+
+            var content = await response.Content.ReadAsStringAsync(ct);
+            return System.Text.Json.JsonSerializer.Deserialize<TaxFiling>(
+                content,
+                new System.Text.Json.JsonSerializerOptions { PropertyNameCaseInsensitive = true });
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogError(ex, "Failed to update filing {FilingId}", id);
+            throw;
+        }
+    }
+
+    public async Task<bool> DeleteAsync(int id, CancellationToken ct = default)
+    {
+        try
+        {
+            var response = await _http.DeleteAsync($"tax-filing/{id}", cancellationToken: ct);
+            return response.IsSuccessStatusCode;
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogError(ex, "Failed to delete filing {FilingId}", id);
+            throw;
+        }
+    }
+
+    private class TaxFilingListResponse
+    {
+        public List<TaxFiling> Data { get; set; } = [];
+    }
+}