feat(样式): 添加移动端适配样式文件并引入

添加 mobile.css 文件实现移动端样式优化，并在 index.php 中通过媒体查询引入
主要针对屏幕宽度小于768px的设备进行布局调整，包括表单元素、按钮、容器等组件的响应式设计

index.php

@@ -105,6 +105,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>内容投稿系统</title>
     <link href="https://cdn.bootcdn.net/ajax/libs/font-awesome/6.0.0/css/all.min.css" rel="stylesheet">
+    <link rel="stylesheet" href="mobile.css" media="only screen and (max-width: 768px)">
     <style>
         * {
             margin: 0;

mobile.css

@@ -0,0 +1,90 @@
+/* mobile.css - 前台投稿页面移动端优化样式 */
+@media (max-width: 768px) {
+  body {
+    padding: 5px;
+    font-size: 16px;
+  }
+  .container {
+    max-width: 100vw;
+    margin: 0;
+    border-radius: 0;
+    box-shadow: none;
+    padding: 0 2px;
+  }
+  .header {
+    padding: 20px 5px;
+    font-size: 1.2rem;
+  }
+  .header h1 {
+    font-size: 1.5rem;
+  }
+  .form-container {
+    padding: 10px 2px !important;
+  }
+  .form-group {
+    margin-bottom: 15px;
+  }
+  .form-group label {
+    font-size: 1rem;
+    margin-bottom: 4px;
+  }
+  .form-group input,
+  .form-group textarea,
+  .form-group select {
+    font-size: 16px;
+    padding: 10px 8px;
+    border-radius: 8px;
+  }
+  .form-group textarea {
+    min-height: 60px;
+  }
+  .btn,
+  .btn-primary,
+  .btn-secondary {
+    width: 100%;
+    font-size: 18px;
+    padding: 14px 0;
+    margin-bottom: 10px;
+    border-radius: 8px;
+  }
+  .btn-group {
+    flex-direction: column;
+    gap: 0;
+  }
+  .tab-container {
+    flex-direction: column;
+    border-radius: 0;
+  }
+  .tab {
+    padding: 14px 0;
+    font-size: 1rem;
+    border-bottom-width: 2px;
+  }
+  .form-section {
+    padding: 0;
+  }
+  .checkbox-group {
+    flex-direction: column;
+    gap: 8px;
+  }
+  .platform-warning {
+    font-size: 13px;
+    padding: 8px;
+  }
+  .message {
+    font-size: 1rem;
+    padding: 10px;
+    border-radius: 8px;
+  }
+  .pagination {
+    flex-wrap: wrap;
+    gap: 5px;
+  }
+  .modal-content {
+    width: 98vw;
+    max-width: 100vw;
+    left: 50%;
+    transform: translate(-50%, -50%);
+    padding: 10px;
+  }
+}