Convert admin pages to new Tailwind design pattern

- Replace 'card' class with 'admin-card' (gradient cards, dark mode support)
- Replace 'muted' class with consistent Tailwind muted states
- Standardize page headers with gradient backgrounds and Lucide icons
- Convert all inline styles to Tailwind classes
- Add admin-page CSS helpers (admin-card, admin-filter-bar, admin-section-title)
- Apply pattern to all 42 admin pages (was only 6 before)
- Zero TypeScript errors, production builds successfully

Co-authored-by: openhands <[email protected]>

src/app/admin/achievements/page.tsx

@@ -1,3 +1,4 @@
+import { Award } from "lucide-react";
 import { StatusCard } from "@/components/admin/dashboard";
 import { prisma } from "@/lib/prisma";
 
@@ -45,10 +46,15 @@ export default async function AdminAchievements() {
 
   return (
     <main>
-      <h1>Achievements</h1>
-      <p className="muted" style={{ marginTop: "-0.4rem" }}>
-        Read-only · {achievements.length} rows across {groups.size} categories.
-      </p>
+      <div className="flex items-center gap-3 mb-6">
+        <div className="w-10 h-10 rounded-xl bg-gradient-to-br from-[var(--color-primary)]/20 to-[var(--color-primary)]/5 grid place-items-center">
+          <Award size={20} className="text-[var(--color-primary)]" />
+        </div>
+        <div>
+          <h1 className="m-0 text-xl font-extrabold text-[var(--color-text)]">Achievements</h1>
+          <p className="m-0 text-xs text-[var(--color-text-muted)] mt-0.5">Read-only · {achievements.length} rows across {groups.size} categories.</p>
+        </div>
+      </div>
 
       <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Achievement rows" value={achievements.length} icon="🏆" />
@@ -57,15 +63,15 @@ export default async function AdminAchievements() {
       </div>
 
       {groups.size === 0 ? (
-        <p className="muted">No achievements found.</p>
+        <div className="admin-empty">No achievements found.</div>
       ) : (
         [...groups.entries()].map(([category, rows]) => (
           <section key={category} className="mt-6">
-            <h2>
+            <h2 className="admin-section-title">
               {category}{" "}
               <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-[var(--color-primary)]/18 text-[var(--color-text)]">{rows.length}</span>
             </h2>
-            <div className="card" style={{ padding: 0, overflowX: "auto" }}>
+            <div className="admin-card p-0 overflow-x-auto">
               <table>
                 <thead>
                   <tr>