Implement Firebase Google sign-in with enum-based auth contracts.

Refactor AuthService to use AuthProvider and User, add Firebase-backed auth wiring for login/manage flows, and fix app-level Google services configuration so Credential Manager sign-in works reliably.

Made-with: Cursor

sdk/build.gradle.kts

@@ -2,7 +2,6 @@ plugins {
     alias(libs.plugins.android.library)
     alias(libs.plugins.ksp)
     alias(libs.plugins.apollo)
-    alias(libs.plugins.google.services)
 }
 
 android {
@@ -50,6 +49,7 @@ dependencies {
     // Firebase
     implementation(platform(libs.firebase.bom))
     implementation(libs.firebase.analytics)
+    implementation(libs.firebase.auth)
     implementation(libs.firebase.crashlytics)
     implementation(libs.firebase.messaging)