fix(ui): 强制浅色主题与登录页细节修正

- 主题：将 CSS 变量默认值由深色改为 ClassicLight 浅色基准（背景/文字/边框/按钮/输入框/滚动条），追加 prefers-color-scheme: dark 下的 color-scheme: light 覆盖，补齐 surface-xxx 别名避免部分组件底色色值缺失
- 语言按钮：删除首页顶栏语言切换按钮及其点击处理逻辑
- 第三方登录：OAuthButtons 重写为使用语义化 CSS 变量而非无效的 --border-color，新增 alwaysShow 选项在未配置 provider 时显示占位提示；登录页强制启用提示
- 页脚与登录页：全部文字颜色与分割线由硬编码 rgba(255*) 改为 var(--text-tertiary/quaternary/border-primary)，在浅色背景下保证对比度

app/components/Auth/LoginForm.vue

@@ -143,7 +143,7 @@
       </button>
     </div>
 
-    <AuthOAuthButtons v-if="!isBindMode" />
+    <AuthOAuthButtons v-if="!isBindMode" :always-show="true" />
 
     <div class="form-footer">
       <p class="help-text">不同VoiceHub平台的账号不互通</p>

app/components/Auth/OAuthButtons.vue

@@ -1,15 +1,13 @@
 <template>
-  <div v-if="hasEnabledProviders" class="mt-6 w-full">
-    <div class="flex items-center text-center text-[var(--text-tertiary)] text-xs mb-4">
-      <div class="flex-1 border-b border-[var(--border-color)] opacity-20" />
-      <span class="px-2.5">或使用第三方账号登录</span>
-      <div class="flex-1 border-b border-[var(--border-color)] opacity-20" />
+  <div v-if="hasEnabledProviders" class="oauth-buttons-section">
+    <div class="oauth-divider">
+      <span>或使用第三方账号登录</span>
     </div>
-    <div class="flex justify-center gap-4">
+    <div class="oauth-buttons">
       <button
         v-if="config.public.oauth.github"
         type="button"
-        class="w-12 h-12 rounded-full border border-[var(--border-color)] bg-[var(--bg-secondary)] text-[var(--text-primary)] flex items-center justify-center cursor-pointer transition-all duration-200 hover:-translate-y-0.5 hover:shadow-sm hover:bg-[var(--bg-tertiary)] hover:bg-[#24292e] hover:text-white hover:border-[#24292e]"
+        class="oauth-btn oauth-btn-github"
         title="使用 GitHub 登录"
         @click="loginWith('github')"
       >
@@ -19,7 +17,7 @@
       <button
         v-if="config.public.oauth.google"
         type="button"
-        class="w-12 h-12 rounded-full border border-[var(--border-color)] bg-[var(--bg-secondary)] text-[var(--text-primary)] flex items-center justify-center cursor-pointer transition-all duration-200 hover:-translate-y-0.5 hover:shadow-sm hover:bg-[var(--bg-tertiary)] hover:bg-white hover:text-black hover:border-[#dadce0]"
+        class="oauth-btn oauth-btn-google"
         title="使用 Google 登录"
         @click="loginWith('google')"
       >
@@ -29,7 +27,7 @@
       <button
         v-if="config.public.oauth.aggregate"
         type="button"
-        class="w-12 h-12 rounded-full border border-[var(--border-color)] bg-[var(--bg-secondary)] text-[var(--text-primary)] flex items-center justify-center cursor-pointer transition-all duration-200 hover:-translate-y-0.5 hover:shadow-sm hover:bg-[var(--bg-tertiary)] hover:bg-[#12B7F5] hover:text-white hover:border-[#12B7F5]"
+        class="oauth-btn oauth-btn-aggregate"
         title="使用 QQ / 微信登录"
         @click="loginWith('aggregate')"
       >
@@ -39,16 +37,158 @@
       </button>
     </div>
   </div>
+
+  <div v-else-if="showPlaceholderHint" class="oauth-buttons-section">
+    <div class="oauth-divider oauth-divider-dim">
+      <span>第三方登录</span>
+    </div>
+    <div class="oauth-empty-hint">
+      <svg
+        class="oauth-empty-icon"
+        fill="none"
+        stroke="currentColor"
+        stroke-width="2"
+        viewBox="0 0 24 24"
+      >
+        <path d="M13 10V3L4 14h7v7l9-11h-7z" />
+      </svg>
+      <p>请联系管理员在 <code>.env</code> 中配置第三方 OAuth 应用后即可启用</p>
+    </div>
+  </div>
 </template>
 
 <script setup lang="ts">
+interface Props {
+  alwaysShow?: boolean
+}
+const props = withDefaults(defineProps<Props>(), {
+  alwaysShow: false
+})
 const config = useRuntimeConfig()
 
 const hasEnabledProviders = computed(() => {
   return config.public.oauth && Object.values(config.public.oauth).some((enabled) => enabled)
 })
 
+const showPlaceholderHint = computed(() => props.alwaysShow && !hasEnabledProviders.value)
+
 const loginWith = (provider: string) => {
   navigateTo(`/api/auth/${provider}`, { external: true })
 }
 </script>
+
+<style scoped>
+.oauth-buttons-section {
+  width: 100%;
+  margin-top: 24px;
+}
+
+.oauth-divider {
+  display: flex;
+  align-items: center;
+  text-align: center;
+  margin: 0 0 18px 0;
+  color: var(--text-quaternary, #9ca3af);
+  font-size: 12px;
+}
+
+.oauth-divider::before,
+.oauth-divider::after {
+  content: '';
+  flex: 1;
+  border-bottom: 1px solid var(--border-secondary, #d1d5db);
+  opacity: 0.85;
+}
+
+.oauth-divider span {
+  padding: 0 12px;
+  white-space: nowrap;
+}
+
+.oauth-divider-dim {
+  color: var(--text-quaternary, #9ca3af);
+  opacity: 0.8;
+}
+
+.oauth-buttons {
+  display: flex;
+  justify-content: center;
+  gap: 16px;
+}
+
+.oauth-btn {
+  width: 48px;
+  height: 48px;
+  border-radius: 999px;
+  border: 1px solid var(--border-secondary, #d1d5db);
+  background: var(--bg-secondary, #f5f7fa);
+  color: var(--text-primary, #1a2332);
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  cursor: pointer;
+  transition:
+    transform 0.2s ease,
+    box-shadow 0.2s ease,
+    background-color 0.2s ease,
+    border-color 0.2s ease,
+    color 0.2s ease;
+}
+
+.oauth-btn:hover {
+  transform: translateY(-1px);
+  box-shadow: var(--shadow-sm);
+}
+
+.oauth-btn-github:hover {
+  background: #24292e;
+  color: #ffffff;
+  border-color: #24292e;
+}
+
+.oauth-btn-google:hover {
+  background: #ffffff;
+  color: #111111;
+  border-color: #dadce0;
+}
+
+.oauth-btn-aggregate:hover {
+  background: #12b7f5;
+  color: #ffffff;
+  border-color: #12b7f5;
+}
+
+.oauth-empty-hint {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 10px;
+  padding: 14px 16px;
+  border: 1px dashed var(--border-secondary, #d1d5db);
+  border-radius: var(--radius-lg);
+  background: var(--bg-tertiary, #edf0f5);
+  color: var(--text-tertiary, #6b7280);
+  font-size: 13px;
+}
+
+.oauth-empty-icon {
+  width: 18px;
+  height: 18px;
+  opacity: 0.8;
+  flex-shrink: 0;
+}
+
+.oauth-empty-hint p {
+  margin: 0;
+  line-height: 1.5;
+}
+
+.oauth-empty-hint code {
+  background: var(--input-bg, #ffffff);
+  color: var(--primary, #1e40af);
+  padding: 1px 6px;
+  border-radius: 4px;
+  font-family: 'Monaco', 'Menlo', 'Ubuntu Mono', monospace;
+  font-size: 12px;
+}
+</style>

app/components/SiteFooter.vue

@@ -96,7 +96,7 @@ onMounted(() => {
   text-align: center;
   padding: 20px 0;
   margin-top: 30px;
-  border-top: 1px solid rgba(255, 255, 255, 0.1);
+  border-top: 1px solid var(--border-primary);
   width: 100%;
 }
 
@@ -109,7 +109,7 @@ onMounted(() => {
 }
 
 .footer-item {
-  color: rgba(255, 255, 255, 0.6);
+  color: var(--text-tertiary);
   font-size: 12px;
   white-space: nowrap;
 }
@@ -117,7 +117,7 @@ onMounted(() => {
 .footer-item:not(:last-child)::after {
   content: ' | ';
   margin: 0 10px;
-  color: rgba(255, 255, 255, 0.4);
+  color: var(--text-quaternary);
 }
 
 .footer-item a {
@@ -127,7 +127,7 @@ onMounted(() => {
 }
 
 .footer-item a:hover {
-  color: rgba(255, 255, 255, 0.8);
+  color: var(--text-secondary);
 }
 
 .icp-link,
@@ -139,7 +139,7 @@ onMounted(() => {
 
 .icp-link:hover,
 .voicehub-link:hover {
-  color: rgba(255, 255, 255, 0.8);
+  color: var(--text-secondary);
 }
 
 .netlify-badge {