feat: JWT 토큰 기반 Admin 로그인 인증 완성

- AuthService로 JWT 토큰 생성 및 검증
- CustomAuthenticationStateProvider를 통한 Blazor 인증 통합
- LocalStorageService로 토큰 관리
- Login.razor 완전 재작성 (실제 DB 검증, 토큰 발급)
- BCrypt 기반 비밀번호 검증
- admin/admin123으로 테스트 가능

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

TaxBaik.Admin/Program.cs

@@ -1,18 +1,18 @@
 using System.Text.Encodings.Web;
 using System.Text.Unicode;
-using Microsoft.AspNetCore.Authentication.Cookies;
+using Microsoft.AspNetCore.Components.Authorization;
 using MudBlazor.Services;
+using TaxBaik.Admin.Services;
 using TaxBaik.Application;
 using TaxBaik.Infrastructure;
 
 var builder = WebApplication.CreateBuilder(args);
 
-builder.Services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
-    .AddCookie(opts => {
-        opts.LoginPath = "/login";
-        opts.ExpireTimeSpan = TimeSpan.FromHours(8);
-        opts.Cookie.SameSite = SameSiteMode.Lax;
-    });
+builder.Services.AddScoped<AuthService>();
+builder.Services.AddScoped<CustomAuthenticationStateProvider>();
+builder.Services.AddScoped<AuthenticationStateProvider>(sp => sp.GetRequiredService<CustomAuthenticationStateProvider>());
+builder.Services.AddScoped<ILocalStorageService, LocalStorageService>();
+builder.Services.AddCascadingAuthenticationState();
 builder.Services.AddAuthorizationCore();
 
 builder.Services.AddRazorComponents()