feat: add undo/redo/reset editor tools with keyboard shortcuts

- Add history extension to CodeMirror for undo/redo support
- Ctrl+Z for undo, Ctrl+Shift+Z for redo now work
- Add toolbar buttons: ↶ Undo, ↷ Redo, ⟲ Reset
- Reset button restores editor to initial lesson code
- Add .btn-icon and .editor-tools CSS styles

src/app.js

@@ -23,6 +23,9 @@ const elements = {
 	taskInstruction: document.getElementById("task-instruction"),
 	codeInput: document.getElementById("code-input"),
 	runBtn: document.getElementById("run-btn"),
+	undoBtn: document.getElementById("undo-btn"),
+	redoBtn: document.getElementById("redo-btn"),
+	resetCodeBtn: document.getElementById("reset-code-btn"),
 	hintArea: document.getElementById("hint-area"),
 	validationIndicators: document.querySelector(".validation-indicators-container"),
 	editorContent: document.querySelector(".editor-content"),
@@ -376,6 +379,19 @@ function prevLesson() {
 
 // ================= CODE EXECUTION =================
 
+function resetCode() {
+	// Reset editor to initial code for current lesson
+	lessonEngine.reset();
+	const engineState = lessonEngine.getCurrentState();
+	if (codeEditor && engineState.lesson) {
+		codeEditor.setValue(engineState.lesson.initialCode || "");
+	}
+	// Clear hints and success indicators
+	clearHint();
+	resetSuccessIndicators();
+	elements.validationIndicators.innerHTML = "";
+}
+
 function runCode() {
 	const userCode = codeEditor ? codeEditor.getValue() : "";
 
@@ -561,6 +577,15 @@ function init() {
 	elements.nextBtn.addEventListener("click", nextLesson);
 	elements.runBtn.addEventListener("click", runCode);
 
+	// Editor tools
+	elements.undoBtn.addEventListener("click", () => {
+		if (codeEditor) codeEditor.undo();
+	});
+	elements.redoBtn.addEventListener("click", () => {
+		if (codeEditor) codeEditor.redo();
+	});
+	elements.resetCodeBtn.addEventListener("click", resetCode);
+
 	// Modals
 	elements.helpBtn.addEventListener("click", showHelp);
 	elements.modalClose.addEventListener("click", closeModal);

src/impl/CodeEditor.js

@@ -3,7 +3,8 @@
  */
 import { EditorState, Prec } from "@codemirror/state";
 import { EditorView, keymap, placeholder } from "@codemirror/view";
-import { defaultKeymap, indentMore, indentLess } from "@codemirror/commands";
+import { defaultKeymap, historyKeymap, indentMore, indentLess, undo, redo } from "@codemirror/commands";
+import { history } from "@codemirror/commands";
 import { oneDark } from "@codemirror/theme-one-dark";
 import { html } from "@codemirror/lang-html";
 import { css } from "@codemirror/lang-css";
@@ -49,6 +50,8 @@ export class CodeEditor {
 			langExtension,
 			oneDark,
 			editorTheme,
+			// History for undo/redo
+			history(),
 			// Emmet abbreviation tracking
 			abbreviationTracker(),
 			// High priority keymap for Emmet
@@ -58,8 +61,9 @@ export class CodeEditor {
 					run: expandAbbreviation
 				}
 			])),
-			// Standard keymaps
+			// Standard keymaps including history (Ctrl+Z, Ctrl+Shift+Z)
 			keymap.of([
+				...historyKeymap,
 				{ key: "Tab", run: indentMore },
 				{ key: "Shift-Tab", run: indentLess },
 				...defaultKeymap
@@ -138,6 +142,24 @@ export class CodeEditor {
 		}
 	}
 
+	/**
+	 * Undo last change
+	 */
+	undo() {
+		if (this.view) {
+			undo(this.view);
+		}
+	}
+
+	/**
+	 * Redo last undone change
+	 */
+	redo() {
+		if (this.view) {
+			redo(this.view);
+		}
+	}
+
 	/**
 	 * Destroy the editor
 	 */

src/index.html

@@ -42,6 +42,11 @@
 							<div class="editor-header">
 								<label for="code-input" class="editor-label">CSS Editor</label>
 								<div class="editor-actions">
+									<div class="editor-tools">
+										<button id="undo-btn" class="btn btn-icon" title="Undo (Ctrl+Z)">↶</button>
+										<button id="redo-btn" class="btn btn-icon" title="Redo (Ctrl+Shift+Z)">↷</button>
+										<button id="reset-code-btn" class="btn btn-icon" title="Reset to initial code">⟲</button>
+									</div>
 									<div class="validation-indicators-container"></div>
 									<button id="run-btn" class="btn btn-run">
 										<img src="./gear.svg" alt="" />Run

src/main.css

@@ -712,6 +712,27 @@ code, kbd {
 	font-size: 0.8rem;
 }
 
+.btn-icon {
+	padding: 4px 8px;
+	font-size: 1rem;
+	min-width: 32px;
+	background: transparent;
+	color: var(--light-text);
+	border: 1px solid var(--border-color);
+}
+
+.btn-icon:hover {
+	background: var(--bg-color);
+	color: var(--text-color);
+	border-color: var(--primary-color);
+}
+
+.editor-tools {
+	display: flex;
+	gap: 4px;
+	margin-right: var(--spacing-sm);
+}
+
 .btn-ghost {
 	background: transparent;
 	color: var(--light-text);