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

build.gradle.kts

@@ -8,4 +8,5 @@ plugins {
     alias(libs.plugins.apollo) apply false
     alias(libs.plugins.hilt.android) apply false
     alias(libs.plugins.google.services) apply false
+    alias(libs.plugins.firebase.crashlytics) apply false
 }