Scope PhoneNumberField trunk-0 strip to prefixes that use a national trunk zero

[marassteiner]

Summary

PhoneNumberField.updatePhoneNumber() strips a single leading 0 from the national part for every prefix. That is correct for the prefixes this field currently offers (+41, +49). It becomes wrong if the field later accepts prefixes whose leading 0 is a significant national digit (for example Italian +39 landlines).

Evidence

• lib/widgets/form/phone_number_field.dart:48 — number!.startsWith('0') ? number!.substring(1) : number with no prefix check
• lib/widgets/form/phone_number_field.dart:16 — current prefixes are only ['+41', '+49']
• Parallel work on feat/open-phone-country-prefix already lets the user type a free 1–3 digit dial code. Combining that with this strip without a country allow-list would sign and store a mutated number.

Found while reviewing #910.

Suggested follow-up

• Restrict the strip to prefixes that use a national trunk zero (at least +41 / +49; +43 / +423 if those stay in the known list).
• When the open-prefix field lands, either keep a trunk-0 allow-list or reuse the same normalisation the API uses.
• Add a preservation test for a prefix that must keep a leading 0 (for example +39 / 06…).

Not a merge blocker for #910: on today's +41/+49 surface the strip is the #905 fix and is covered by widget tests.