fix(ui): restore message loading position and slow hint

[ChristophWurst]

Before

Bildschirmaufzeichnung.vom.2026-07-03.16-23-40.mp4

After, with fixed position

Bildschirmaufzeichnung.vom.2026-07-03.16-24-35.mp4

I was skeptical if this could break the scroll to load more feature but it still works 😌

After, with slow hint

Bildschirmaufzeichnung.vom.2026-07-03.16-46-38.mp4

How to test

For me it was easiest to force drop the local cache entries for my testing account, then reload the page and navigate to another mailbox:

DELETE FROM oc_mail_messages m
WHERE mailbox_id IN (SELECT id FROM oc_mail_mailboxes WHERE account_id = <id>);
UPDATE oc_mail_mailboxes SET sync_new_lock = null, sync_new_token = null, sync_changed_lock = null, sync_changed_token = null, sync_vanished_lock = null, sync_vanished_token = null
WHERE account_id = <id>;

Note: there is a bug that the loading never stops. That's why I started looking into this. Mandatory reference for this situation: https://www.youtube.com/watch?v=AbSehcT19u0.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[ChristophWurst]

/backport to stable5.10