fix: login button stuck on 준비 중 - Blazor hydration reverted JS enable

AdminLoginForm's submit button had disabled hardcoded as static markup, not
bound to component state. The early inline <script> (before WASM boots)
flipped it via raw DOM mutation, but when the WASM runtime later resumed the
prerendered component, Blazor's own first render re-asserted the static
disabled from the markup - silently undoing the JS fix. The second
bindLoginForm() call from OnAfterRenderAsync then bailed out immediately on
the one-shot "already bound" guard, so nothing ever re-enabled it.

Fix: bind disabled to a real isReady field flipped in OnAfterRenderAsync so
Blazor owns that attribute going forward, and make the JS-side enable
idempotent (runs on every call, not gated behind the bind-once guard) as a
second line of defense.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

TaxBaik.Web.Client/Components/Admin/Shared/AdminLoginForm.razor

@@ -29,10 +29,10 @@
 
             <button type="submit"
                     id="admin-login-submit"
-                    disabled
+                    disabled="@(!isReady)"
                     class="mud-button-root mud-button mud-button-filled mud-button-filled-primary mud-elevation-0"
                     style="width: 100%; min-height: 52px; border: 0; border-radius: 4px; color: white;">
-                <span>준비 중...</span>
+                <span>@(isReady ? "로그인" : "준비 중...")</span>
             </button>
         </form>
     </MudPaper>
@@ -40,6 +40,7 @@
 
 @code {
     private string rememberedUsername = "";
+    private bool isReady;
     private const string RememberedUsernameKey = "admin-remembered-username";
 
     protected override async Task OnInitializedAsync()
@@ -67,6 +68,15 @@
             {
                 // Login UI must remain visible even if JS binding fails.
             }
+            finally
+            {
+                // Blazor owns this render from here on, so drive "disabled" from
+                // C# state rather than a raw DOM mutation - otherwise this hydration
+                // pass re-asserts the prerendered markup's static "disabled" and
+                // silently undoes whatever the early inline <script> already set.
+                isReady = true;
+                StateHasChanged();
+            }
         }
     }
 }