Fix compilation warnings after Kotlin 2.4.20 and KSP 2.3.12 upgrade

- Update .idea/kotlinc.xml to Kotlin 2.4.20.
- Remove `-Xannotation-default-target=first-only`: the new annotation
  default target rule is stable in Kotlin 2.4, and disabling it now
  produces a CLI_ARG_DISABLES_STABLE_FEATURE warning.
- Replace deprecated `KSNode.validate()` with
  `validate(enableNewFeatures = false)` to keep the same behavior.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

build.gradle.kts

@@ -101,10 +101,6 @@ allprojects {
 
             // Uncomment to suppress Compose Kotlin compiler compatibility warning
 //            freeCompilerArgs.addAll(listOf("-P", "plugin:androidx.compose.compiler.plugins.kotlin:suppressKotlinVersionCompatibilityCheck=true"))
-
-            // Fix compilation warning for annotations
-            // See https://youtrack.jetbrains.com/issue/KT-73255/Change-defaulting-rule-for-annotations for more details
-            freeCompilerArgs.add("-Xannotation-default-target=first-only")
         }
     }
 }