feat: tier names + per-tier prices on offer cards, unified admin UI

- offer editor: per-tier name input (mirrors public_name/internal_name)
- offer list cards: show 3-tier services total + manual public price
- shared PageHeader component, full-width layout across all admin pages
- UI-RULES.md design conventions doc

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/admin/PageHeader.tsx

@@ -0,0 +1,17 @@
+import type { ReactNode } from "react";
+
+export function PageHeader({ title, subtitle, action }: {
+  title: string;
+  subtitle?: string;
+  action?: ReactNode;
+}) {
+  return (
+    <div className="mb-6 flex items-start justify-between gap-4">
+      <div>
+        <h1 className="text-2xl font-bold text-[#1a1a1a]">{title}</h1>
+        {subtitle && <p className="mt-1 text-sm text-[#71717a]">{subtitle}</p>}
+      </div>
+      {action}
+    </div>
+  );
+}