refactor: merge playground into welcome module

- Rename welcome lessons to Get Started, Overview, Playground
- Remove separate playground module files
- Fix playground detection to check lesson.mode instead of module.id

src/app.js

@@ -390,7 +390,7 @@ function loadCurrentLesson() {
 
 	const lesson = engineState.lesson;
 	const mode = lesson.mode || engineState.module?.mode || "css";
-	const isPlayground = engineState.module.id === "playground";
+	const isPlayground = lesson.mode === "playground";
 
 	// Handle playground mode - hide instructions, full height editor
 	if (isPlayground) {
@@ -581,7 +581,7 @@ function resetCode() {
 function runCode() {
 	const userCode = codeEditor ? codeEditor.getValue() : "";
 	const engineState = lessonEngine.getCurrentState();
-	const isPlayground = engineState.module?.id === "playground";
+	const isPlayground = engineState.lesson?.mode === "playground";
 
 	// Rotate the Run button icon
 	const runButtonImg = document.querySelector("#run-btn img");

src/config/lessons.js

@@ -19,7 +19,6 @@ import htmlTablesEN from "../../lessons/30-html-tables.json";
 import htmlMarqueeEN from "../../lessons/31-html-marquee.json";
 import htmlSvgEN from "../../lessons/32-html-svg.json";
 import flexboxEN from "../../lessons/flexbox.json";
-import playgroundEN from "../../lessons/99-playground.json";
 
 // German lesson imports
 import welcomeDE from "../../lessons/de/00-welcome.json";
@@ -37,7 +36,6 @@ import htmlTablesDE from "../../lessons/de/30-html-tables.json";
 import htmlMarqueeDE from "../../lessons/de/31-html-marquee.json";
 import htmlSvgDE from "../../lessons/de/32-html-svg.json";
 import flexboxDE from "../../lessons/de/flexbox.json";
-import playgroundDE from "../../lessons/de/99-playground.json";
 
 // English module store - ordered by learning path
 const moduleStoreEN = [
@@ -62,9 +60,7 @@ const moduleStoreEN = [
 	flexboxEN,
 	responsiveEN,
 	// CSS Animationen
-	transitionsAnimationsEN,
-	// Playground
-	playgroundEN
+	transitionsAnimationsEN
 ];
 
 // German module store - ordered by learning path
@@ -90,9 +86,7 @@ const moduleStoreDE = [
 	flexboxDE,
 	responsiveDE,
 	// CSS Animationen
-	transitionsAnimationsDE,
-	// Playground
-	playgroundDE
+	transitionsAnimationsDE
 ];
 
 /**