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

BACKLOG.md

@@ -20,7 +20,7 @@ bottom as they come up.
 - [x] Keyboard-shortcut help overlay (press `?`), listing space / arrows / f / m.
 - [x] Download/preload progress indicator on cards (percent or a bar, not just a pulse).
 - [x] Remember and restore the last playback position per video.
-- [ ] Toast queue (stack multiple toasts instead of replacing).
+- [x] Toast queue (stack multiple toasts instead of replacing).
 
 ## Features
 - [ ] Drag-to-reorder videos within a playlist.