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

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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