Add Container component and fix radius/maxWidth theming

- Drop buttonRadius prop; button now uses --radius via rounded-md
- Inject @theme radius mappings into ThemeProvider so rounded-* utilities
  pick up --radius inside the Tailwind CDN iframe
- Add shared Container that consumes --container-max-width set from the
  global maxWidth prop, replacing ad-hoc "container mx-auto max-w-7xl px-6"
  wrappers across commerce, landing, footer, navigation, and others
- Simplify maxWidth options to Small/Medium/Large/X-Large/Full bleed and
  shift the scale up so Large (1280px) matches the previous default

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

components/layout/Container.tsx

@@ -0,0 +1,21 @@
+import * as React from "react";
+import { cn } from "@/lib/utils";
+
+export type ContainerProps = React.HTMLAttributes<HTMLElement> & {
+  as?: React.ElementType;
+};
+
+export function Container({
+  as: Comp = "div",
+  className,
+  style,
+  ...props
+}: ContainerProps) {
+  return (
+    <Comp
+      className={cn("mx-auto w-full px-6", className)}
+      style={{ maxWidth: "var(--container-max-width, 80rem)", ...style }}
+      {...props}
+    />
+  );
+}