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.

app/not-found.tsx

@@ -14,9 +14,11 @@ export default function NotFound() {
       </div>
 
       <div className="text-center px-4 z-10">
-        <h1 className="text-9xl font-bold text-transparent bg-clip-text bg-gradient-to-b from-white to-neutral-500 mb-4">
-          404
-        </h1>
+        <img
+          src="/illustrations/page-not-found.svg"
+          alt="Page not found"
+          className="w-72 h-auto mx-auto mb-8"
+        />
         <h2 className="text-2xl font-bold text-white mb-6">
           Page not found
         </h2>

app/welcome/page.tsx

@@ -39,8 +39,6 @@ import {
   ArrowRightIcon,
   ArrowLeftIcon,
   BarChartIcon,
-  GlobeIcon,
-  ZapIcon,
   PlusIcon,
 } from '@ciphera-net/ui'
 import Link from 'next/link'
@@ -440,9 +438,11 @@ function WelcomeContent() {
                 </>
               ) : (
                 <div className="text-center">
-                  <div className="mx-auto flex h-14 w-14 items-center justify-center rounded-full bg-brand-orange/10 text-brand-orange mb-6">
-                    <ZapIcon className="h-7 w-7" />
-                  </div>
+                  <img
+                    src="/illustrations/welcome.svg"
+                    alt="Welcome to Pulse"
+                    className="w-48 h-auto mx-auto mb-6"
+                  />
                   <h1 className="text-2xl font-bold text-neutral-900 dark:text-white">
                     Welcome to Pulse
                   </h1>
@@ -638,9 +638,11 @@ function WelcomeContent() {
                 Back
               </button>
               <div className="text-center mb-6">
-                <div className="mx-auto flex h-14 w-14 items-center justify-center rounded-full bg-brand-orange/10 text-brand-orange mb-4">
-                  <GlobeIcon className="h-7 w-7" />
-                </div>
+                <img
+                  src="/illustrations/website-setup.svg"
+                  alt="Add your first site"
+                  className="w-44 h-auto mx-auto mb-4"
+                />
                 <h1 className="text-2xl font-bold text-neutral-900 dark:text-white">
                   Add your first site
                 </h1>
@@ -723,9 +725,11 @@ function WelcomeContent() {
               className={cardClass}
             >
               <div className="text-center">
-                <div className="mx-auto flex h-14 w-14 items-center justify-center rounded-full bg-green-500/10 text-green-600 dark:text-green-400 mb-6">
-                  <CheckCircleIcon className="h-7 w-7" />
-                </div>
+                <img
+                  src="/illustrations/confirmed.svg"
+                  alt="All set"
+                  className="w-44 h-auto mx-auto mb-6"
+                />
                 <h1 className="text-2xl font-bold text-neutral-900 dark:text-white">
                   You're all set
                 </h1>