Original PR description

**Steps to reproduce:**
1. Create a draft Swiss ELM payslip for an hourly-paid employee with no automatic hourly work-entry/input
2. In the Wages tab, manually update the `Factor` (`rate`) field of the `Hourly Salary` line
3. Save the payslip

**Issue:**
The manually entered `Factor` is reset to `0.00`

**Cause:**
- `l10n_ch_swiss_wage_ids` is a stored computed field without explicit `readonly=False`, causing manual modifications to the line values to be discarded during field recomputation on save.

opw-6536243