feat: live IMAP sync on folder select + 60s auto-sync interval

- New imap_sync_folder function: syncs single folder quickly for live use
- New POST /mail/folders/{folder_id}/sync route
- Frontend: syncFolder API + called on every folder selection
- Auto-sync interval reduced from 300s to 60s
- Sent mail IMAP APPEND already uploads full body via msg.as_bytes()

frontend/src/api/mail.ts

@@ -481,6 +481,10 @@ export function emptyFolder(folderId: string): Promise<{ emptied_count: number }
   return apiPost<{ emptied_count: number }>(`/mail/folders/${folderId}/empty`, {});
 }
 
+export function syncFolder(folderId: string): Promise<{ synced: number; error?: string }> {
+  return apiPost<{ synced: number; error?: string }>(`/mail/folders/${folderId}/sync`, {});
+}
+
 // ─── Mails ──────────────────────────────────────────────────────────────────
 
 export function fetchMails(folderId: string, page: number, sortBy?: string, sortOrder?: string): Promise<MailListResult> {