Port viewer-theme from production Lit composer, fix card bg and heading margins

- Add viewerTheme.ts: exact port of the production viewer-theme used by
  A2UIViewer on a2ui-composer.ag-ui.com (from private_a2ui_demo repo)
- Add appTheme.ts: the app-level theme used for chat view (for future use)
- Switch adapter to use viewerTheme instead of composerTheme
- Fix card background in gallery: define --a2ui-card-bg at :root level
  and reference it via --p-100 so the gallery wrapper can override it
  to transparent (matching Lit's themed-a2ui-surface behavior)
- Fix heading margins: add layout-m-0 to markdown.h4 and markdown.h5
  to prevent browser default margins on h4/h5 elements

Author: lukasmoschitz

tools/composer/src/app/globals.css

@@ -66,6 +66,7 @@
 }
 
 :root {
+  --a2ui-card-bg: #ffffff;
   --radius: 0.625rem;
   --background: oklch(1 0 0);
   --foreground: oklch(0.145 0 0);
@@ -182,7 +183,7 @@
   --n-0: #000000;
 
   /* Primary palette (indigo) */
-  --p-100: #ffffff;
+  --p-100: var(--a2ui-card-bg);
   --p-99: #fffbff;
   --p-98: #fcf8ff;
   --p-95: #f2efff;

tools/composer/src/lib/a2ui.tsx

@@ -13,12 +13,12 @@
 
 import { A2UIViewer as BaseA2UIViewer } from "@a2ui/react";
 import type { ComponentProps } from "react";
-import { composerTheme } from "./composerTheme";
+import { viewerTheme } from "./viewerTheme";
 
 export type { ComponentInstance } from "@a2ui/react";
 
 type A2UIViewerProps = ComponentProps<typeof BaseA2UIViewer>;
 
 export function A2UIViewer(props: A2UIViewerProps) {
-  return <BaseA2UIViewer theme={composerTheme} {...props} />;
+  return <BaseA2UIViewer theme={viewerTheme} {...props} />;
 }