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>

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.14.2-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.4.1-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME