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/main.css

@@ -1154,6 +1154,9 @@ button.lesson-list-item {
 }
 
 .btn-icon {
+	display: flex;
+	align-items: center;
+	justify-content: center;
 	padding: 4px 8px;
 	font-size: 1rem;
 	min-width: 32px;
@@ -1168,6 +1171,13 @@ button.lesson-list-item {
 	border-color: var(--primary-color);
 }
 
+.btn-icon img {
+	width: 1rem;
+	height: 1rem;
+	margin: 0;
+	display: block;
+}
+
 .editor-tools {
 	display: flex;
 	gap: 4px;