feat(portal): 고객 포털 인증과 소셜 로그인 기반 추가

TaxBaik.Web/Pages/Portal/Login.cshtml

@@ -0,0 +1,40 @@
+@page "/portal/login"
+@model TaxBaik.Web.Pages.Portal.LoginModel
+@{
+    ViewData["Title"] = "고객 포털 로그인";
+    ViewData["Description"] = "고객 포털 로그인 페이지입니다.";
+    ViewData["CanonicalUrl"] = $"{Request.Scheme}://{Request.Host}/taxbaik/portal/login";
+}
+
+<section class="container py-5" style="max-width: 560px;">
+    <h1 class="h3 fw-bold mb-4">고객 포털 로그인</h1>
+    <div class="alert alert-secondary">
+        포털 인증은 다음 단계에서 이메일/비밀번호와 소셜 로그인으로 연결됩니다.
+    </div>
+    @if (!string.IsNullOrWhiteSpace(Model.ErrorMessage))
+    {
+        <div class="alert alert-danger">@Model.ErrorMessage</div>
+    }
+    <form method="post" class="vstack gap-3">
+        <div>
+            <label class="form-label">이메일</label>
+            <input class="form-control" asp-for="Email" />
+        </div>
+        <div>
+            <label class="form-label">비밀번호</label>
+            <input class="form-control" asp-for="Password" type="password" />
+        </div>
+        <button class="btn btn-dark" type="submit">로그인</button>
+    </form>
+    <div class="d-grid gap-2 mt-4">
+        <form method="post" asp-page-handler="Google">
+            <button class="btn btn-outline-dark w-100" type="submit">Google로 로그인</button>
+        </form>
+        <form method="post" asp-page-handler="Naver">
+            <button class="btn btn-outline-success w-100" type="submit">Naver로 로그인</button>
+        </form>
+        <form method="post" asp-page-handler="Kakao">
+            <button class="btn btn-outline-warning w-100" type="submit">Kakao로 로그인</button>
+        </form>
+    </div>
+</section>