session-studio: fix repositories block + pin Java 21 in gradle.properties

- build.gradle: add repositories { mavenCentral() } (fixes org.json resolve error)
- gradle.properties: pin org.gradle.java.home to openjdk-bin-21 (Gradle 8.9 unsupported on Java 25)

Co-authored-by: Cursor <cursoragent@cursor.com>

build.gradle

@@ -3,6 +3,10 @@ plugins {
     id 'org.openjfx.javafxplugin' version '0.1.0'
 }
 
+repositories {
+    mavenCentral()
+}
+
 java {
     toolchain {
         languageVersion = JavaLanguageVersion.of(21)