refactor: allow providers to use whatever sign-in methods they wish

[russellwheatley]

Type Safety Improvements

• Introduced CredentialAuthProviderSwift protocol to distinguish credential-based providers from other auth flows
• Provider initializers now accept concrete types (e.g., GoogleProviderSwift) instead of generic protocols
• Eliminated force-casting throughout the codebase

Phone Auth Simplification

• Removed PhoneAuthProviderSwift protocol - phone verification logic now handled internally by AuthService
• Simplified phone auth button initialization (no longer requires provider parameter)
• Phone reauthentication now explicitly requires sign-out/sign-in flow

@demolaf - not possible to move phone auth methods out of auth service package because the methods are used in the Views of this package.

[russellwheatley]

@demolaf - one of the reasons why I liked phone auth being a separate flow is because now when the user needs to reauthenticate for sensitive operations (e.g. changing password), we have to throw error asking the user to sign-out and sign back in again because pushing user to enter phone number will clear the View they're in already and state would be lost.

[demolaf]

LGTM