Update/deps and toolchain

[PrivateBasicsApp]

1. JCenter is shut down and no longer serves the Kotlin/AGP plugins, so resolve dependencies from Maven Central instead — this fixes the "Could not resolve kotlin-gradle-plugin" build failure. Also move to Gradle 8.7 / AGP 8.5.2, drop the unused Jetifier, and suppress the compileSdk 36 warning.
2. Bump gson (→2.11.0), okhttp (→4.12.0), commons, and Material (off the 1.2.0 beta), plus appcompat/core/fragment/lifecycle and the test libraries to the newest versions still compatible with Kotlin 1.8. Guard a startActivity() intent that the fragment update turned non-null.
3. Drop the mutable tourguide SNAPSHOT dependency together with ShowcaseBuilder and its four tooltip call sites. Define the White color locally, since it was previously provided by tourguide's bundled resources.
4. Request POST_NOTIFICATIONS at runtime so notifications appear on Android 13+, remove the obsolete manifest package attribute (the namespace is set in Gradle), and mark the runtime-registered receiver as not-exported.