A running task/ticket timer can sometimes stutter, show negative values, and track the wrong elapsed time.

### Steps to reproduce

On a record with a timesheet timer (for example, a Field Service task):

1. Start the timer and let it run for about 15-20 seconds.
2. Stop it and confirm the dialog.
3. Start it again. This creates a new `timer_start`.
4. Reload the page.

The timer starts jumping every second between two different values. As it keeps running, it can even show negative values such as `00:00:-57`.

If the problem does not appear right away, repeat steps 2-4 a few times. It usually shows up after a few stop/start/reload cycles.

### Cause

The timer shown in the button bar is the `timer_start_field` widget. It starts a `setInterval` that updates a shared `TimerReactive` object once per second.

While a form is loading, Odoo renders it several times in a row (for example: a first render, another when the chatter is loaded, and another when the record data comes back from the server). Rendering a form builds all of its fields to produce the display, so each of these renders creates its own `timer_start_field`. Odoo keeps and mounts only the render that ends up on screen; the earlier ones are thrown away before being mounted.

The interval is started while the field renders, from the record observer set up in `setup`, before the field is mounted. So the fields that are later thrown away also start an interval. Those intervals keep running for the rest of the session. Each one updates the same shared `TimerReactive` object using the `timer_start` it was created with.

As long as every instance has the same `timer_start`, they all write the same value and the problem stays hidden. After the timer is stopped and started again, the old instances keep the old `timer_start` while the mounted one uses the new one. Every second they overwrite each other's value, so the timer jumps between two different elapsed times. When the instance with the newer `timer_start` writes right after one with an older start, it tries to show a smaller elapsed time than what is already there, and the subtraction in `TimerReactive` produces a negative number of seconds.

### Fix

Move the per-second timer update into a `useEffect`. The effect only runs after the field is mounted, and Owl automatically cleans it up when the field is unmounted or when `timer_start` or `timer_pause` change.

This means fields that are destroyed before they are mounted never start an interval, so only the mounted field updates the shared timer.

`onRecordChange` no longer starts or stops the interval. It only updates the displayed timer value to match the current record.

opw-6209405