feat: improve playground UX and fix undo/redo across lessons

- Add dice SVG icon for random template button
- Reset button now restores last loaded template in playground
- Clear editor history when switching lessons (prevents cross-lesson undo)
- Add playground link to goodbye lesson
- Center icon buttons with flexbox

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

src/impl/CodeEditor.js

@@ -183,7 +183,7 @@ export class CodeEditor {
 	}
 
 	/**
-	 * Set editor value
+	 * Set editor value (preserves history)
 	 */
 	setValue(value) {
 		if (!this.view) return;
@@ -197,6 +197,13 @@ export class CodeEditor {
 		});
 	}
 
+	/**
+	 * Set editor value and clear history (for lesson switching)
+	 */
+	setValueAndClearHistory(value) {
+		this.init(value);
+	}
+
 	/**
 	 * Set editor mode (html or css)
 	 */