Timer save fails silently when current month is locked

[alxhall]

Description
When the current month is locked and a user attempts to save a running timer, the POST request to Milltime returns HTTP 404 "timer not found". The timer continues running with no user facing error message or feedback. This behavior is identical in both the web timer and the TUI.

Steps to reproduce

1. Lock the current month in Milltime
2. Start a timer
3. Attempt to save the timer
   → Timer keeps running, no error is shown

Additional observations

• Manually setting the entry's start date to the next month works fine.
• Project registration also fails between 00:00–01:00 on the first day of the new month (likely due to UTC offset), and recovers automatically after 01:00.

Expected behavior
The API should detect that the month is locked and return a meaningful error. Both the TUI and the web timer should surface this error to the user (e.g. display an appropriate message and clear/stop the running timer).

Notes
This may be a Milltime limitation (feature?) rather than a bug, but the silent failure is worth addressing.