fix(docker): skip frontend Gradle tasks when SKIP_FRONTEND is set

Add onlyIf predicate to installFrontendDependencies and buildFrontend tasks
so they are skipped when SKIP_FRONTEND env var is present. This fixes the
Docker multi-stage build which failed because Gradle validated input files
(package.json, package-lock.json) that don't exist in the builder stage.

Also moved the skipFrontend variable definition before task registration
for cleaner code organization.

Co-authored-by: Junie <junie@jetbrains.com>

backend/build.gradle

@@ -35,7 +35,11 @@ def frontendDir = file('../frontend')
 def frontendDistDir = file('../frontend/dist/frontend/browser')
 def springStaticDir = file('src/main/resources/static')
 
+// Skip frontend build tasks when building inside Docker (frontend already built in Stage 1)
+def skipFrontend = project.hasProperty('skipFrontend') || System.getenv('SKIP_FRONTEND') != null
+
 tasks.register('installFrontendDependencies', Exec) {
+	onlyIf { !skipFrontend }
 	workingDir frontendDir
 
 	inputs.file(file('../frontend/package.json'))
@@ -50,6 +54,7 @@ tasks.register('installFrontendDependencies', Exec) {
 }
 
 tasks.register('buildFrontend', Exec) {
+	onlyIf { !skipFrontend }
 	dependsOn 'installFrontendDependencies'
 	workingDir frontendDir
 
@@ -66,9 +71,6 @@ tasks.register('buildFrontend', Exec) {
 	}
 }
 
-// Skip frontend build tasks when building inside Docker (frontend already built in Stage 1)
-def skipFrontend = project.hasProperty('skipFrontend') || System.getenv('SKIP_FRONTEND') != null
-
 tasks.register('copyFrontend', Copy) {
 	onlyIf { !skipFrontend }
 	dependsOn 'buildFrontend'