feat: reuse tabs across multi-run — no more tab bloat

- tabRegistry preserved across resetState (like seenCodes/accounts)
- New reuseOrCreateTab() helper: navigates existing tab or creates new one
- All steps use reuseOrCreateTab: VPS panel, auth page, mail
- Mail tabs: only activate if alive, don't re-navigate (preserves session)
- Auth tab: reused between step 2→6→8, navigated to new URLs
- VPS tab: reused across runs, re-injected on navigation

sidepanel/sidepanel.html

@@ -140,6 +140,7 @@
     <div id="log-area"></div>
   </section>
 
+  <div id="toast-container"></div>
   <script src="sidepanel.js"></script>
 </body>
 </html>