refactor: migrate contact form to TanStack Form

- Replace manual state management with @tanstack/react-form
- Add proper Field/FieldLabel components for accessibility
- Simplify server action (remove useActionState signature)
- Remove use-debounce dependency
- Update PGP key links and minor styling tweaks

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -47,6 +47,7 @@
     "@radix-ui/react-toggle-group": "^1.1.11",
     "@radix-ui/react-tooltip": "^1.2.8",
     "@t3-oss/env-nextjs": "^0.13.10",
+    "@tanstack/react-form": "^1.28.0",
     "@vercel/analytics": "^1.6.1",
     "@vercel/functions": "^3.4.0",
     "@vercel/speed-insights": "^1.3.1",
@@ -98,7 +99,6 @@
     "tailwind-merge": "^3.4.0",
     "tailwindcss": "^4.1.18",
     "unified": "^11.0.5",
-    "use-debounce": "^10.1.0",
     "zod": "^4.3.6"
   },
   "devDependencies": {