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/libs.versions.toml

@@ -1,7 +1,7 @@
 [versions]
-agp = "8.10.1"
-kotlin = "2.1.20"
-ksp = "2.1.20-1.0.32"
+agp = "9.1.0"
+kotlin = "2.3.20"
+ksp = "2.3.6"
 compose-bom = "2026.03.00"
 room = "2.8.4"
 navigation = "2.9.7"