feat: new onboarding screens (#4)

Changelog:
- Add a custom splash screen
- Redesign the onboarding screen
- Improve UX for the new and import identity screens

Reviewed-on: https://git.reya.su/reya/coop-mobile/pulls/4

composeApp/src/androidMain/kotlin/su/reya/coop/MainActivity.kt

@@ -6,10 +6,13 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+import androidx.core.splashscreen.SplashScreen.Companion.installSplashScreen
 
 class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
+        installSplashScreen()
         enableEdgeToEdge()
+
         super.onCreate(savedInstanceState)
 
         val intent = Intent(this, NostrForegroundService::class.java)