Bug: normalize comparison chart progression range across currencies #111

Summary:
Adding a high-nominal currency country like Sweden stretches the comparison chart gross range far beyond the intended baseline.

Reproduction:

1. Open the calculator comparison view.
2. Compare countries with synced salaries in mixed currencies, for example US, CH, GR, PT, IT, then add Sweden.
3. Open the progression chart.

Actual:
The chart upper bound jumps to values derived from the highest local-currency gross amount, for example SEK, which then gets reused for every country. This makes the net salary chart run into multi-million gross ranges that do not reflect the shared baseline.

Expected:
The progression chart range should be normalized to a shared baseline currency, and each country should be evaluated at its own local-currency gross equivalent for each x-axis point.

Notes:
Current implementation appears to derive the progression range from raw gross_annual form values and send the same numeric progression points to all countries, even when those countries use different currencies.