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/app/admin/impostazioni/page.tsx

@@ -1,6 +1,7 @@
 import { getTargetHourlyRate, updateSetting, SETTINGS_KEYS } from "@/lib/settings";
 import { getAllPools } from "@/lib/taxonomy";
 import { TaxonomyManager } from "@/components/admin/impostazioni/TaxonomyManager";
+import { PageHeader } from "@/components/admin/PageHeader";
 
 export const revalidate = 0;
 
@@ -16,8 +17,8 @@ export default async function ImpostazioniPage() {
   }
 
   return (
-    <div>
-      <h1 className="text-2xl font-bold text-[#1a1a1a] mb-6">Impostazioni</h1>
+    <div className="space-y-6">
+      <PageHeader title="Impostazioni" />
 
       <div className="space-y-6 max-w-4xl">
         <div className="bg-white rounded-xl border border-[#e5e7eb] p-6 max-w-md">