[Bug]: unclean handling of SimpleFIN downtime

### Verified issue does not already exist?

- [X] I have searched and found no existing issue
- [X] I will be providing steps how to reproduce the bug (in most cases this will also mean uploading a demo budget file)

### What happened?

Trying to sync banks and I get the error "This account is experiencing connection problems. Let’s fix it." Clicking that give the details "An internal error occurred. Try to login again, or get [in touch](https://actualbudget.org/contact/) for support." and the option to unlink the account. I didn't try it but i think this would also fail.

Using this test script https://gist.github.com/duplaja/4df7e47230566894b7ee2a7e45ac2f50 also errors, and the "pickle" file contains HTML with title tag "Temporary Maintenance - SimpleFIN Bridge" and a messages section stating "This website is undergoing temporary maintenance. Some functions are unavailable at this time."

Ideally, Actual would check the SimpleFIN return and show that message instead of offering to unlink.

### Where are you hosting Actual?

Docker

### What browsers are you seeing the problem on?

Chrome

### Operating System

Windows 11

Start by running the linked SimpleFIN reproduction script and inspect how its response is handled when it contains the “Temporary Maintenance - SimpleFIN Bridge” HTML. Trace the sync error path that produces the connection-problems message and unlink option. Done means temporary SimpleFIN downtime is reported using the returned maintenance message instead of being presented as an internal error or unlink action.

Scritto dal modello di indicizzazione a partire dal testo della issue.