Width, instead of a negative number to match by hand

Inside the band, whole width of main, or whole page width. The negatives are
gone from the padding select — padding is padding again.

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

web/canvas.ts

@@ -55,6 +55,8 @@ export type CanvasComponent = {
   kind: string;
   /** The heading the page draws above the block, or null. Every kind may have one. */
   title?: string | null;
+  /** `main`, `page` or null — how wide it is allowed to be. See the site's schema. */
+  width?: string | null;
   /*
     One optional field per kind that the panel reads back. Not `Record<string, unknown>`: every reader
     would then need a cast, and the point of naming them is that the compiler knows `free-text` holds