Replace single toast with stacked toast queue

Multiple toasts can appear simultaneously, capped at 3. Oldest
toast auto-evicts when cap is reached. Each toast auto-dismisses
with a fade-up exit animation.

- frontend/app.js: toast() now creates DOM elements in toastContainer
- frontend/index.html: single #toast replaced with #toastContainer
- frontend/styles.css: .toast-container flexbox row, .toast-exit animation

frontend/index.html

@@ -162,7 +162,7 @@
       </div>
     </div>
 
-    <div id="toast" class="toast hidden"></div>
+    <div id="toastContainer" class="toast-container"></div>
 
     <!-- Keyboard shortcuts help overlay -->
     <div id="shortcutHelp" class="shortcut-overlay hidden">