fix: Pass javaResourceFolder to Options in DevModeInitializer (#23440)

DevModeInitializer was creating an Options object without setting
javaResourceFolder, causing TaskUpdateSettingsFile to overwrite it
with an empty string. This broke legacy theme component styles in
Gradle dev mode because the Vite theme plugin couldn't find
vaadin-featureflags.properties in the correct location.

Fixes #23438

Author: Artur-

vaadin-dev-server/src/main/java/com/vaadin/base/devserver/startup/DevModeInitializer.java

@@ -225,7 +225,8 @@ public static DevModeHandler initDevModeHandler(Set<Class<?>> classes,
                 .withFrontendDirectory(frontendFolder)
                 .withFrontendGeneratedFolder(
                         new File(frontendFolder + FrontendUtils.GENERATED))
-                .withBuildDirectory(config.getBuildFolder());
+                .withBuildDirectory(config.getBuildFolder())
+                .setJavaResourceFolder(config.getJavaResourceFolder());
 
         log().info("Starting dev-mode updaters in {} folder.",
                 options.getNpmFolder());