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.js

@@ -19,6 +19,8 @@ const btnAutoRun = document.getElementById('btn-auto-run');
 const btnAutoContinue = document.getElementById('btn-auto-continue');
 const autoContinueBar = document.getElementById('auto-continue-bar');
 const btnClearLog = document.getElementById('btn-clear-log');
+const inputVpsUrl = document.getElementById('input-vps-url');
+const selectMailProvider = document.getElementById('select-mail-provider');
 
 // ============================================================
 // State Restore on load
@@ -39,6 +41,12 @@ async function restoreState() {
     if (state.email) {
       inputEmail.value = state.email;
     }
+    if (state.vpsUrl) {
+      inputVpsUrl.value = state.vpsUrl;
+    }
+    if (state.mailProvider) {
+      selectMailProvider.value = state.mailProvider;
+    }
 
     if (state.stepStatuses) {
       for (const [step, status] of Object.entries(state.stepStatuses)) {
@@ -238,7 +246,7 @@ btnClearLog.addEventListener('click', () => {
   logArea.innerHTML = '';
 });
 
-// Save email on change
+// Save settings on change
 inputEmail.addEventListener('change', async () => {
   const email = inputEmail.value.trim();
   if (email) {
@@ -246,6 +254,20 @@ inputEmail.addEventListener('change', async () => {
   }
 });
 
+inputVpsUrl.addEventListener('change', async () => {
+  const vpsUrl = inputVpsUrl.value.trim();
+  if (vpsUrl) {
+    await chrome.runtime.sendMessage({ type: 'SAVE_SETTING', source: 'sidepanel', payload: { vpsUrl } });
+  }
+});
+
+selectMailProvider.addEventListener('change', async () => {
+  await chrome.runtime.sendMessage({
+    type: 'SAVE_SETTING', source: 'sidepanel',
+    payload: { mailProvider: selectMailProvider.value },
+  });
+});
+
 // ============================================================
 // Listen for Background broadcasts
 // ============================================================