refactor(phone-number): convert phoneNumber and selectedCountry to signals

[spike-rabbit]

This is a preparation so we can convert other fields to computed later in a follow-up PR.

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[gemini-code-assist]

Code Review

This pull request refactors the SiPhoneNumberInputComponent to use Angular Signals for state management, converting selectedCountry and phoneNumber into signals and updating the template and component logic accordingly. The review feedback focuses on ensuring proper reactivity and safety when working with these signals. Specifically, it recommends configuring the phoneNumber signal with a custom equality check to handle in-place mutations of the mutable PhoneNumber class, extracting signals into local variables to avoid multiple reads and unsafe non-null assertions, and explicitly calling .set() after mutating the phone number object to trigger reactivity.

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Already implemented