fix: incremental IMAP sync - only fetch new UIDs since last known, like real mail clients

- imap_sync_folder now searches only UID > max_known_uid instead of ALL
- First sync gets last 50, subsequent syncs only get new mails
- UI loads cached mails immediately from DB, sync runs in background
- Folder counts updated after sync

frontend/src/pages/Mail.tsx

@@ -169,7 +169,7 @@ export function MailPage() {
     loadMails();
   }, [loadMails]);
 
-  // Handle folder selection — derive account from folder + trigger live IMAP sync
+  // Handle folder selection — derive account from folder + trigger background IMAP sync
   const handleSelectFolder = useCallback(
     (folderId: string) => {
       setSelectedFolderId(folderId);
@@ -182,7 +182,7 @@ export function MailPage() {
       setSelectedMail(null);
       setSelectedMailIds(new Set());
       setActiveView('list');
-      // Live sync: fetch new mails from IMAP server immediately
+      // Live sync: fetch new mails from IMAP in background, reload after
       syncFolder(folderId).then(() => {
         loadMails();
         loadAllFolders();