feat: cache mailbox list for 10 minutes and show progress on initial fetch

- Add 10-minute cache after fetching mailbox list from mail accounts
- Display progress during initial mailbox retrieval
- Prevent timeouts when handling large mailbox lists

crates/memdb/src/db.rs

@@ -310,7 +310,7 @@ impl MemDb {
             }
         }
 
-        eprintln!("[memdb] snapshot saved at seq={last_seq}");
+        //eprintln!("[memdb] snapshot saved at seq={last_seq}");
         Ok(())
     }