Enhancement: Make activity form fields automatically update

#### Describe the task

The activity form fields do not update automatically based on user selection. We have observed the following behaviour:
1. `Equivalent Emission` field does not update unless user clicks `Save` or `Save & Continue`
2. When updating a form that has a `CAS Registry Number` field, the value does not change when Gas Type is changed unless user clicks `Save` or `Save & Continue`

#### Acceptance Criteria

- [ ] first
- [ ] second
- [ ] third

#### Definition of Done

- [ ] first
- [ ] second
- [ ] third

#### Additional context

- Add any other context about the task here.
- Or here

Start by reproducing the activity form behavior for Equivalent Emission and for CAS Registry Number after changing Gas Type. Trace where those fields receive their values and compare the behavior before and after Save or Save & Continue. Done means both dependent values update immediately after the relevant selection changes, with concrete acceptance criteria and tests added.

Written by the indexing model from the issue text.