fix: Critical runtime bug - TokenRefreshHandler JS interop in Blazor Server

**Problem:**
TokenRefreshHandler (DelegatingHandler) runs on a non-circuit thread.
ILocalStorageService (JS interop) only works during component render.
Production: 401 response → token refresh → JS interop fails silently.

**Solution:**
1. ITokenStore - Scoped in-memory token store (no JS interop)
   - Properties: AccessToken, RefreshToken, TokenExpiryTicks
   - Method: IsAccessTokenExpired()

2. TokenStore implementation
   - Replaces localStorage as primary token source
   - DelegatingHandler reads/writes only to TokenStore
   - Pages reload → GetAuthenticationStateAsync restores from localStorage

3. CustomAuthenticationStateProvider
   - Accepts ITokenStore injection
   - LoginAsync: Write to both TokenStore + localStorage
   - LogoutAsync: Clear both
   - GetAuthenticationStateAsync: Read from TokenStore first, fallback to localStorage

4. AdminDashboardClient BaseAddress fix
   - Was: new Uri("/taxbaik/api/") - relative URI (runtime error)
   - Now: Configured in Program.cs as absolute URI
   - Program.cs: AddHttpClient(..., client => client.BaseAddress = new Uri("http://localhost:5001/taxbaik/api/"))

**Architecture:**
- TokenStore: Scoped in-memory (DelegatingHandler use)
- localStorage: Persistent (page reload recovery)
- Pattern: Server-side token management without JS interop

This fixes the cascading failure that would occur on any 401 in production.

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

TaxBaik.Web/Program.cs

@@ -67,9 +67,13 @@ builder.Services.AddAuthorization();
 builder.Services.AddAuthorizationCore();
 
 // HTTP Client for API (with automatic token refresh)
+builder.Services.AddScoped<ITokenStore, TokenStore>();
 builder.Services.AddScoped<TokenRefreshHandler>();
 builder.Services.AddHttpClient<IApiClient, ApiClient>();
-builder.Services.AddHttpClient<IAdminDashboardClient, AdminDashboardClient>()
+builder.Services.AddHttpClient<IAdminDashboardClient, AdminDashboardClient>(client =>
+{
+    client.BaseAddress = new Uri("http://localhost:5001/taxbaik/api/");
+})
     .AddHttpMessageHandler<TokenRefreshHandler>();
 
 // UI & 캐시