Upgrade to Gradle 9.4.1, AGP 9.1.0, Kotlin 2.3.20

- Gradle wrapper 8.14.2 -> 9.4.1
- AGP 8.10.1 -> 9.1.0 (built-in Kotlin, no separate kotlin.android plugin)
- Kotlin 2.1.20 -> 2.3.20
- KSP 2.1.20-1.0.32 -> 2.3.6 (new versioning scheme)
- settings.gradle.kts: removed @Suppress for dependencyResolutionManagement
- gradle.properties: added android.disallowKotlinSourceSets=false for KSP compat

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

settings.gradle.kts

@@ -10,7 +10,6 @@ plugins {
     id("org.gradle.toolchains.foojay-resolver-convention") version "1.0.0"
 }
 
-@Suppress("UnstableApiUsage")
 dependencyResolutionManagement {
     repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
     repositories {