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()

app/plugins/builtins/mail/plugin.py

@@ -29,7 +29,7 @@ async def _auto_sync_loop() -> None:
             await auto_sync_all_accounts()
         except Exception as exc:
             logger.warning("auto_sync error: %s", exc)
-        await asyncio.sleep(300)
+        await asyncio.sleep(60)
 
 
 class MailPlugin(BasePlugin):