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.

background.js

@@ -33,7 +33,7 @@ const DEFAULT_SETTINGS = {
 
 // 初始化扩展
 chrome.runtime.onInstalled.addListener(async () => {
-  console.log("Xget for Chrome 已安装");
+  console.log("Xget for Chromium 已安装");
 
   // 如果尚未设置，则设置默认设置
   const settings = await chrome.storage.sync.get(DEFAULT_SETTINGS);