feat: configurable VPS URL, 163 mail support, mail provider selector

- VPS URL: now an input field in Side Panel, no longer hardcoded
  - Dynamic script injection via chrome.scripting.executeScript
  - host_permissions changed to <all_urls> for flexibility
- 163 Mail: new content script (mail-163.js) with actual selectors
  - Mail items: div[sign="letter"], sender: .nui-user, subject: span.da0
  - Supports aria-label fallback for matching
- Mail provider selector: dropdown to switch between QQ Mail and 163 Mail
  - Background routes steps 4/7 to correct mail content script
- Settings persist in chrome.storage.session

content/utils.js

@@ -2,11 +2,12 @@
 
 const SCRIPT_SOURCE = (() => {
   const url = location.href;
-  if (url.includes('154.26.182.181')) return 'vps-panel';
   if (url.includes('auth0.openai.com') || url.includes('auth.openai.com') || url.includes('accounts.openai.com')) return 'signup-page';
   if (url.includes('mail.qq.com')) return 'qq-mail';
+  if (url.includes('mail.163.com')) return 'mail-163';
   if (url.includes('chatgpt.com')) return 'chatgpt';
-  return 'unknown';
+  // VPS panel — detected dynamically since URL is configurable
+  return 'vps-panel';
 })();
 
 const LOG_PREFIX = `[MultiPage:${SCRIPT_SOURCE}]`;