feat: add illustrations to 404, error page, and welcome flow

- 404 page: replace large "404" text with page-not-found illustration
- ErrorDisplay: replace warning icon with server-down illustration
- Welcome step 1 (no orgs): welcome illustration
- Welcome step 4 (add site): website-setup illustration
- Welcome step 5 (done): confirmed illustration
All SVGs dark-themed with brand orange accent.

components/ErrorDisplay.tsx

@@ -30,11 +30,11 @@ export default function ErrorDisplay({
       </div>
 
       <div className="text-center px-4 z-10">
-        <div className="inline-flex items-center justify-center w-16 h-16 rounded-2xl bg-red-100 dark:bg-red-900/30 mb-6">
-          <svg className="w-8 h-8 text-red-500" fill="none" viewBox="0 0 24 24" strokeWidth={1.5} stroke="currentColor">
-            <path strokeLinecap="round" strokeLinejoin="round" d="M12 9v3.75m-9.303 3.376c-.866 1.5.217 3.374 1.948 3.374h14.71c1.73 0 2.813-1.874 1.948-3.374L13.949 3.378c-.866-1.5-3.032-1.5-3.898 0L2.697 16.126ZM12 15.75h.007v.008H12v-.008Z" />
-          </svg>
-        </div>
+        <img
+          src="/illustrations/server-down.svg"
+          alt="Something went wrong"
+          className="w-56 h-auto mx-auto mb-8"
+        />
 
         <h2 className="text-2xl font-bold text-neutral-900 dark:text-white mb-4">
           {title}