fix(auth): 다크 폼 입력·비밀번호 토글 스타일 보정

- .auth-form-input 전역 클래스(글자색·캐럿·placeholder·autofill)
- 토글 버튼 scoped CSS로 고정, signup 패널 보더·배경·color-scheme
- v0.0.62 문서 반영

Co-authored-by: Cursor <cursoragent@cursor.com>

pages/signin.vue

@@ -51,7 +51,7 @@ const submitSignIn = async () => {
 </script>
 
 <template>
-  <section class="auth-signin min-h-screen bg-[#0a0b0d] text-[#f5f7fa]">
+  <section class="auth-signin min-h-screen bg-[#0a0b0d] text-[#f5f7fa] [color-scheme:dark]">
     <div class="mx-auto flex min-h-screen w-full max-w-[1280px] items-center px-5 py-12 sm:px-10 lg:px-16">
       <div class="w-full max-w-[430px] p-5 sm:p-8">
         <p class="text-2xl font-semibold leading-tight">
@@ -66,7 +66,7 @@ const submitSignIn = async () => {
             <label class="text-xs text-[#d8dee6]">이메일</label>
             <input
               v-model="form.email"
-              class="h-10 w-full rounded-[8px] border border-[#1a212a] bg-transparent px-3 text-sm outline-none transition-colors focus:border-[#2f6feb]"
+              class="auth-form-input h-10 w-full rounded-[8px] border border-[#1a212a] bg-transparent px-3 text-sm outline-none transition-colors focus:border-[#2f6feb]"
               type="email"
               autocomplete="email"
             >
@@ -77,7 +77,7 @@ const submitSignIn = async () => {
             <div class="flex items-center rounded-[8px] border border-[#1a212a] transition-colors focus-within:border-[#2f6feb]">
               <input
                 v-model="form.password"
-                class="h-10 min-w-0 flex-1 bg-transparent px-3 text-sm outline-none"
+                class="auth-form-input h-10 min-w-0 flex-1 bg-transparent px-3 text-sm outline-none"
                 :type="showPassword ? 'text' : 'password'"
                 autocomplete="current-password"
               >