fix: Provider button shape customization and theming API (#2288)

* feat: introduce provider button shape customization and update theming API

* feat: add back navigation support to authentication screens

* feat: add customizable screen transitions for Firebase Auth UI

* feat: update theming API to support null theme inheritance and introduce adaptive theme

* refactor: simplify theme observation in AuthUIThemeTest by using CompositionLocalProvider

---------

Co-authored-by: Rosário P. Fernandes <[email protected]>

Author: demolaf

app/src/main/java/com/firebaseui/android/demo/CustomSlotsThemingDemoActivity.kt

@@ -5,6 +5,8 @@ import android.util.Log
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+import androidx.compose.animation.slideInHorizontally
+import androidx.compose.animation.slideOutHorizontally
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Spacer
@@ -13,6 +15,7 @@ import androidx.compose.foundation.layout.height
 import androidx.compose.material3.Button
 import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.ShapeDefaults
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
@@ -24,6 +27,7 @@ import androidx.compose.ui.unit.dp
 import com.firebase.ui.auth.AuthException
 import com.firebase.ui.auth.AuthState
 import com.firebase.ui.auth.FirebaseAuthUI
+import com.firebase.ui.auth.configuration.AuthUITransitions
 import com.firebase.ui.auth.configuration.PasswordRule
 import com.firebase.ui.auth.configuration.authUIConfiguration
 import com.firebase.ui.auth.configuration.auth_provider.AuthProvider
@@ -42,12 +46,23 @@ class HighLevelApiDemoActivity : ComponentActivity() {
         val authUI = FirebaseAuthUI.getInstance()
         val emailLink = intent.getStringExtra(EmailLinkConstants.EXTRA_EMAIL_LINK)
 
+        val customTheme = AuthUITheme.Default.copy(
+            providerButtonShape = ShapeDefaults.ExtraLarge
+        )
+
         val configuration = authUIConfiguration {
             context = applicationContext
+            theme = customTheme
             logo = AuthUIAsset.Resource(R.drawable.firebase_auth)
             tosUrl = "https://policies.google.com/terms"
             privacyPolicyUrl = "https://policies.google.com/privacy"
             isAnonymousUpgradeEnabled = false
+            transitions = AuthUITransitions(
+                enterTransition = { slideInHorizontally { it } },
+                exitTransition = { slideOutHorizontally { -it } },
+                popEnterTransition = { slideInHorizontally { -it } },
+                popExitTransition = { slideOutHorizontally { it } }
+            )
             providers {
                 provider(AuthProvider.Anonymous)
                 provider(