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/applications/page.tsx

@@ -1,4 +1,5 @@
 import Link from "next/link";
+import { ClipboardList } from "lucide-react";
 import { dismissApplication } from "@/actions/admin-applications";
 import { StatusCard } from "@/components/admin/dashboard";
 import { prisma } from "@/lib/prisma";