Allow page width to be overridden

packages/bbui/src/Layout/Page.svelte

@@ -1,8 +1,9 @@
 <script>
   export let wide = false
+  export let maxWidth = "80ch"
 </script>
 
-<div class:wide>
+<div style="--max-width: {maxWidth}" class:wide>
   <slot />
 </div>
 
@@ -12,7 +13,7 @@
     flex-direction: column;
     justify-content: flex-start;
     align-items: stretch;
-    max-width: 80ch;
+    max-width: var(--max-width);
     margin: 0 auto;
     padding: calc(var(--spacing-xl) * 2);
     min-height: calc(100% - var(--spacing-xl) * 4);