fix: frontend race condition — folder sync callback was reloading wrong folder

- When user switches folders quickly, syncFolder(A) callback fires after user already selected B
- loadMails() was called with stale selectedFolderId, loading wrong mails
- Add useRef to track current folder ID, only reload if still on same folder
- Also fix backend: message_id dedup now only within same folder (not cross-folder)

frontend/src/pages/Mail.tsx

@@ -4,7 +4,7 @@
  * Account selection is integrated into the folder tree (left pane).
  */
 
-import React, { useState, useEffect, useCallback } from 'react';
+import React, { useState, useEffect, useCallback, useRef } from 'react';
 import { useTranslation } from 'react-i18next';
 import { Link } from 'react-router-dom';
 import { Card } from '@/components/ui/Card';
@@ -82,6 +82,12 @@ export function MailPage() {
   const [sortOrder, setSortOrder] = useState<'asc' | 'desc'>('desc');
   const [isSyncing, setIsSyncing] = useState(false);
 
+  // Ref to track the current folder ID for async callbacks (prevents race conditions)
+  const selectedFolderIdRef = useRef<string | null>(null);
+  useEffect(() => {
+    selectedFolderIdRef.current = selectedFolderId;
+  }, [selectedFolderId]);
+
   // Load accounts
   const loadAccounts = useCallback(async () => {
     setLoadingAccounts(true);
@@ -183,9 +189,12 @@ export function MailPage() {
       setSelectedMailIds(new Set());
       setActiveView('list');
       // Live sync: fetch new mails from IMAP in background, reload after
+      // Only reload if the user hasn't switched to another folder in the meantime
       syncFolder(folderId).then(() => {
-        loadMails();
-        loadAllFolders();
+        if (selectedFolderIdRef.current === folderId) {
+          loadMails();
+          loadAllFolders();
+        }
       }).catch(() => {});
     },
     [folders, loadMails, loadAllFolders],