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/index.html

@@ -245,7 +245,7 @@
 										>
 											⟲
 										</button>
-										<button id="random-template-btn" class="btn btn-icon hidden" title="Load random template">🎲</button>
+										<button id="random-template-btn" class="btn btn-icon hidden" title="Load random template"><img src="./dice.svg" alt="" /></button>
 									</div>
 									<button id="run-btn" class="btn btn-run"><img src="./gear.svg" alt="" /><span data-i18n="run">Run</span></button>
 								</div>