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>

app/build.gradle.kts

@@ -1,6 +1,5 @@
 plugins {
     alias(libs.plugins.android.application)
-    alias(libs.plugins.kotlin.android)
     alias(libs.plugins.kotlin.compose)
     alias(libs.plugins.ksp)
 }
@@ -42,8 +41,6 @@ android {
         warningsAsErrors = true
         abortOnError = true
         checkDependencies = true
-        // AGP 9.x requires Gradle 9.x; suppress until we're ready to migrate
-        disable += "AndroidGradlePluginVersion"
     }
 }