HFI Calculator: Usability - don't display success toast (changes saved) if no changes made

**Describe the task**
Presently if I visit the HFI Calculator and click on a fire start dropdown with a default 0-1 fire starts value, open the dropdown, then click away, the page reloads and I get a 'changes saved' success toast. But nothing changed.
Users rely on this feature to verify something was changed - it is confusing if no values were modified (defeats purpose / dilutes impact).

**Acceptance Criteria**
- [ ] a success toast only displays if a value was changed for all inputs: fuel, starts

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

Start in the HFI Calculator flow and reproduce the fire-start dropdown interaction described in the issue. Trace how fuel and starts inputs determine whether the page reloads and the success toast appears. Done means the toast appears only when a fuel or starts value actually changes, including when dismissing an unchanged dropdown.

Written by the indexing model from the issue text.