fix(expo): support themed SSO secondary buttons

[swolfand]

Summary

• Add a secondaryButton color token to Android UI theming and resolve it through light/dark/default theme colors.
• Use secondaryButton for SSO/social and standard secondary button fills.
• Derive secondary button text color from the secondary fill so dark auth backgrounds can still use white SSO buttons with readable labels.
• Make the auth development-mode footer use the themed background instead of a hardcoded white background.

Linear

MOBILE-528

Validation

• ./gradlew spotlessApply
• ./gradlew :source:ui:compileDebugKotlin
• ./gradlew :source:ui:testDebugUnitTest --tests 'com.clerk.ui.theme.ClerkThemeProviderTest'
• ./gradlew :source:api:publishToMavenLocal :source:ui:publishToMavenLocal with signAllPublications() temporarily commented and then restored
• Reinstalled the Expo native playground on Android and verified the SSO button text/footer visually on device