SlideCanvas — pure-CSS 16:9 wrapper that scales a 1920×1080 stage to fit any canvas

Per @dididecks/shell contract, rendered play decks must be static HTML + CSS — no JS in the scaling pipeline. SlideCanvas uses CSS container queries (`container-type: size`) to compute a unitless scale ratio from canvas size to the design size (1920×1080 by default), then applies `transform: scale()` to a `position: absolute` stage. `min(100cqw/designW, 100cqh/designH)` picks whichever axis is the binding constraint so the slide letterboxes correctly when canvas aspect ≠ 16:9. The route's `[slot].astro` is responsible for sizing the canvas; this component just translates that size into a scaled stage.