feat(auth): add environment-driven theming for text fields, CTA buttons and typography

[demolaf]

• Related to Customizable SwiftUI auth UI: content slots, per-screen state, and theming #1400

Adds environment-driven theming for the SwiftUI auth flow's text fields, CTA buttons and typography. Unset values keep today's exact appearance. First of the PRs splitting up #1400.

Changes

• Add AuthTextFieldStyle, AuthCTAButtonStyle and AuthTypography, each set with a View modifier and carried through the environment so they reach pushed screens inside the auth sheet.
• Replace bare .buttonStyle(.borderedProminent) and .font(_:) calls across the auth screens with .authCTAButtonStyle() and .authFont(_:weight:). Brand-styled provider buttons and the Delete Account button are unchanged.
• Apply AuthTypography to typed text and AuthTextFieldStyle to the verification code boxes, not just AuthTextField.
• Move MFAResolutionView onto VerificationCodeInputField and the shared button styles, so theming reaches it like the other MFA screens.
• Re-export FirebaseAuthUIComponents from FirebaseAuthSwiftUI. It is not a library product, so AuthTextFieldStyle and AuthTypography were otherwise unreachable from import FirebaseAuthSwiftUI.

API Usage

AuthPickerView { authenticatedApp }
  .authTypography(AuthTypography(fontName: "AvenirNext-Regular"))
  .authTextFieldStyle(AuthTextFieldStyle(tint: .purple, cornerRadius: 16))
  .authCTAButtonStyle(AuthCTAButtonStyle(backgroundColor: .purple, shape: .capsule))

Preview

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Maintainer note: Fixes internal CPRN-515

[gemini-code-assist]

Code Review

This pull request introduces a customizable styling and typography system for the authentication flow, including AuthCTAButtonStyle for primary buttons, AuthTextFieldStyle for text fields, and AuthTypography for custom font families. The feedback highlights three key issues: a double-scaling bug in AuthTypography when resolving custom fonts with Dynamic Type, a regression in AuthCTAButtonModifier where passing a nil background color clears the inherited environment tint, and a visual regression in EmailAuthView where the blue color of the switch-auth-flow action label was accidentally removed.