PhoneInput: controllable country + detect country from pasted numbers #31

PhoneInput: controllable country + detect country from pasted numbers

Today the selected country is internal state seeded once from defaultCountry:

const [selectedCountry, setSelectedCountry] = React.useState(() => getCountryByCode(defaultCountry) ?? COUNTRIES[0]);

This breaks the library's controlled/uncontrolled convention (see ARCHITECTURE.md) — a parent can't change or reset the country, and changing defaultCountry after mount does nothing.

Proposal

• Add country / onCountryChange and route both through useControllableState; keep defaultCountry for uncontrolled use.
• When the user types or pastes a number starting with + (e.g. +44 20 7946 0958), match the longest dial code, switch the country and strip the prefix from the input value.
• Shared dial codes (+1 US/CA, +7 RU/KZ): keep the currently selected country if it matches, else pick the first.

Acceptance

• Tests: controlled country follows parent; paste +44… selects GB; +1 keeps CA if CA selected
• Docs updated with props table