fix(mobile-app): resolve Kotlin build errors in build.gradle.kts

- Rename getAutoVersionCode() to calculateNextVersionCode() to avoid
  JVM signature clash with auto-generated property getter
- Remove unnecessary Elvis operator for non-nullable flutter.versionName
- Migrate deprecated kotlinOptions.jvmTarget to kotlin.jvmToolchain DSL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/mobile-app/android/app/build.gradle.kts

@@ -22,7 +22,7 @@ if (keyPropertiesFile.exists()) {
 val versionPropertiesFile = rootProject.file("app/version.properties")
 val versionProperties = Properties()
 
-fun getAutoVersionCode(): Int {
+fun calculateNextVersionCode(): Int {
     if (versionPropertiesFile.exists()) {
         versionProperties.load(FileInputStream(versionPropertiesFile))
     }
@@ -34,12 +34,12 @@ fun getAutoVersionCode(): Int {
 }
 
 // Get auto-incremented version code
-val autoVersionCode = getAutoVersionCode()
+val autoVersionCode = calculateNextVersionCode()
 
 // Version name format: major.minor.patch (from pubspec.yaml) + build number
 // Example: 1.0.0.123
 fun getAutoVersionName(): String {
-    val flutterVersionName = flutter.versionName ?: "1.0.0"
+    val flutterVersionName = flutter.versionName
     return "$flutterVersionName.$autoVersionCode"
 }
 
@@ -53,8 +53,8 @@ android {
         targetCompatibility = JavaVersion.VERSION_17
     }
 
-    kotlinOptions {
+    kotlin {
-        jvmTarget = JavaVersion.VERSION_17.toString()
+        jvmToolchain(17)
     }
 
     defaultConfig {