style(PortalLayout): 将标题和页脚从红色改为黑金主题色

- 顶部公告条背景改为黑色
- 主导航背景改为黑色渐变
- 底线装饰改为金色渐变
- 页脚背景改为黑色，边框改为金色

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

offer_frontend/src/layouts/PortalLayout.vue

@@ -50,7 +50,7 @@
               <button class="btn-ghost">登录</button>
             </router-link>
             <router-link to="/register">
-              <button class="btn-primary">立即注册</button>
+              <button class="btn-ghost">立即注册</button>
             </router-link>
           </template>
           <template v-else>
@@ -118,7 +118,7 @@ a { text-decoration: none; }
 
 /* 顶部公告条 */
 .top-bar {
-  background: #8B4545;
+  background: #1A1A1A;
   color: rgba(255,255,255,0.75);
   font-size: 12px;
   letter-spacing: 0.05em;
@@ -144,7 +144,7 @@ a { text-decoration: none; }
 
 /* 主导航 */
 .main-header {
-  background: linear-gradient(180deg, #C17A7A 0%, #A85555 100%);
+  background: linear-gradient(180deg, #1A1A1A 0%, #0F0F0F 100%);
   box-shadow: 0 4px 20px rgba(0,0,0,0.1);
   position: sticky;
   top: 0;
@@ -271,11 +271,11 @@ a { text-decoration: none; }
 }
 .user-name { color: rgba(255,255,255,0.85); font-size: 13px; }
 
-/* 红色底线装饰 */
+/* 底线装饰 */
-.header-underline { height: 3px; background: #8B4545; }
+.header-underline { height: 3px; background: #0F0F0F; }
 .underline-fill {
   height: 100%;
-  background: linear-gradient(90deg, #FFFFFF 0%, #EF9A9A 40%, #D4A95D 100%);
+  background: linear-gradient(90deg, #C8973A 0%, #D4AF37 100%);
 }
 
 /* 内容区 */
@@ -289,8 +289,8 @@ a { text-decoration: none; }
 
 /* 页脚 */
 .portal-footer {
-  background: #8B4545;
+  background: #1A1A1A;
-  border-top: 3px solid #FFFFFF;
+  border-top: 3px solid #C8973A;
   padding: 20px 0;
 }
 .footer-inner {