Rename extension references from Chrome to Chromium

Updated all documentation, configuration, and code references from 'Xget for Chrome' to 'Xget for Chromium' for consistency with the supported browser family. This includes issue templates, workflow files, privacy and security policies, README, manifest, package metadata, background and content scripts, and popup UI.

popup.html

@@ -3,7 +3,7 @@
   <head>
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
-    <title>Xget for Chrome</title>
+    <title>Xget for Chromium</title>
     <style>
       body {
         width: 380px;
@@ -211,7 +211,7 @@
   </head>
   <body>
     <div class="header">
-      <h1>Xget for Chrome</h1>
+      <h1>Xget for Chromium</h1>
       <div class="subtitle">下载加速器</div>
     </div>
 
@@ -282,7 +282,7 @@
             <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"
+            href="https://github.com/xixu-me/Xget for Chromium"
             target="_blank"
             class="link-item"
           >