Localize documentation and extension messages to Chinese

Converted all major documentation files (README, PRIVACY_POLICY, SECURITY, pull request template) to Chinese. Updated extension source files (background.js, content.js, popup.html, popup.js) to use Chinese comments and user-facing messages for improved localization and user experience.

popup.html

@@ -212,7 +212,7 @@
   <body>
     <div class="header">
       <h1>Xget for Chrome</h1>
-      <div class="subtitle">Download Accelerator</div>
+      <div class="subtitle">下载加速器</div>
     </div>
 
     <div class="content">
@@ -225,7 +225,7 @@
               justify-content: space-between;
             "
           >
-            Enable Extension
+            启用扩展
             <label class="toggle-switch">
               <input type="checkbox" id="enabledToggle" />
               <span class="slider"></span>
@@ -235,19 +235,17 @@
       </div>
 
       <div class="setting-group">
-        <label class="setting-label" for="domainInput"
-          >Xget Domain (Required)</label
-        >
+        <label class="setting-label" for="domainInput">Xget 域名（必填）</label>
         <input
           type="text"
           id="domainInput"
           class="domain-input"
-          placeholder="your-xget-domain.com"
+          placeholder="你的-xget-域名.com"
         />
       </div>
 
       <div class="setting-group">
-        <label class="setting-label">Enabled Platforms</label>
+        <label class="setting-label">启用的平台</label>
         <div class="platforms">
           <div class="platform-item">
             <span>GitHub</span>
@@ -281,7 +279,7 @@
             class="link-item"
           >
             <span class="link-icon">🚀</span>
-            <span class="link-text">Deploy Your Own Xget Service</span>
+            <span class="link-text">部署你自己的 Xget 服务</span>
           </a>
           <a
             href="https://github.com/xixu-me/Xget-for-Chrome"
@@ -289,7 +287,7 @@
             class="link-item"
           >
             <span class="link-icon">📖</span>
-            <span class="link-text">Source Code & Documentation</span>
+            <span class="link-text">源码和文档</span>
           </a>
         </div>
       </div>