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

sidepanel/sidepanel.html

@@ -41,13 +41,24 @@
   <section id="data-section">
     <div class="data-card">
       <div class="data-row">
-        <span class="data-label">OAuth</span>
-        <span id="display-oauth-url" class="data-value mono truncate">Waiting...</span>
+        <span class="data-label">VPS</span>
+        <input type="text" id="input-vps-url" class="data-input" placeholder="http://ip:port/management.html#/oauth" />
+      </div>
+      <div class="data-row">
+        <span class="data-label">Mail</span>
+        <select id="select-mail-provider" class="data-select">
+          <option value="qq">QQ Mail (wx.mail.qq.com)</option>
+          <option value="163">163 Mail (mail.163.com)</option>
+        </select>
       </div>
       <div class="data-row">
         <span class="data-label">Email</span>
         <input type="text" id="input-email" class="data-input" placeholder="Paste DuckDuckGo email" />
       </div>
+      <div class="data-row">
+        <span class="data-label">OAuth</span>
+        <span id="display-oauth-url" class="data-value mono truncate">Waiting...</span>
+      </div>
       <div class="data-row">
         <span class="data-label">Callback</span>
         <span id="display-localhost-url" class="data-value mono truncate">Waiting...</span>