Fix decimal point issue in expense form schema amount field

[jantuomi]

Problem

The amount input field in the "add expense" view uses the decimal inputMode. This renders a numbers-only keyboard that on some locales only allows the user to input a comma as the decimal point. However, the zod schema does not permit a comma as the decimal point, since Number(value) will return NaN for such numbers.

E.g. Number("1.23") === 1.23, but Number("1,23") === NaN.

Solution

Replace commas with dots in the user-inputted amount value before casting it to a JS number. The rest of the schema validation will proceed as normal.

Fixes #78

[scastiel]

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