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

app/build.gradle.kts

@@ -5,6 +5,8 @@ plugins {
     alias(libs.plugins.hilt.android)
     alias(libs.plugins.ksp)
     alias(libs.plugins.kotlin.compose)
+    alias(libs.plugins.google.services)
+    alias(libs.plugins.firebase.crashlytics)
 }
 
 fun gitCommitCount(): String {
@@ -81,6 +83,9 @@ dependencies {
     implementation(libs.androidx.navigation.compose)
     implementation(libs.hilt.android)
     implementation(libs.androidx.hilt.navigation.compose)
+    implementation(libs.androidx.credentials)
+    implementation(libs.androidx.credentials.play.services.auth)
+    implementation(libs.googleid)
     ksp(libs.hilt.compiler)
 
     // Tests