a media block shows what is in it

"Choose media…" and nothing else asks somebody to open a dialog to find out what
they already chose. The thumbnail and the filename answer it where they stand.

A video shows its poster rather than playing: the question is which file this is,
and a frame answers that.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/canvas.ts

@@ -60,7 +60,7 @@ export type CanvasComponent = {
     would then need a cast, and the point of naming them is that the compiler knows `free-text` holds
     `html` and `gallery` holds `galleryId`.
   */
-  content?: { html?: string; galleryId?: number | null };
+  content?: { html?: string; galleryId?: number | null; url?: string };
   padding?: Record<string, { x?: number; y?: number }>;
 };
 export type CanvasSection = {