feat(empty-states): add undraw illustrations to empty state screens

Add brand-orange recolored SVG illustrations from undraw to five empty
states: sites list, dashboard chart, funnels, journeys, and behavior.

components/behavior/FrustrationTable.tsx

@@ -177,9 +177,11 @@ export default function FrustrationTable({
             </>
           ) : (
             <div className="h-full flex flex-col items-center justify-center text-center px-6 py-8 gap-4">
-              <div className="rounded-full bg-neutral-100 dark:bg-neutral-800 p-4">
-                <CursorClick className="w-8 h-8 text-neutral-500 dark:text-neutral-400" />
-              </div>
+              <img
+                src="/illustrations/blank-canvas.svg"
+                alt="No frustration signals"
+                className="w-44 h-auto mb-1"
+              />
               <h4 className="font-semibold text-neutral-900 dark:text-white">
                 No {title.toLowerCase()} detected
               </h4>

components/dashboard/Chart.tsx

@@ -414,7 +414,12 @@ export default function Chart({
           </div>
 
           {!hasData || !hasAnyNonZero ? (
-            <div className="flex h-96 flex-col items-center justify-center gap-2">
+            <div className="flex h-96 flex-col items-center justify-center gap-3">
+              <img
+                src="/illustrations/no-data.svg"
+                alt="No data available"
+                className="w-48 h-auto mb-2"
+              />
               <p className="text-sm text-neutral-400 dark:text-neutral-500">
                 {!hasData ? 'No data for this period' : `No ${METRIC_CONFIGS.find((m) => m.key === metric)?.label.toLowerCase()} recorded`}
               </p>

components/journeys/ColumnJourney.tsx

@@ -556,9 +556,11 @@ export default function ColumnJourney({
   if (!transitions.length) {
     return (
       <div className="h-[400px] flex flex-col items-center justify-center text-center px-6 py-8 gap-3">
-        <div className="rounded-full bg-neutral-100 dark:bg-neutral-800 p-4">
-          <TreeStructure className="w-8 h-8 text-neutral-500 dark:text-neutral-400" />
-        </div>
+        <img
+          src="/illustrations/journey.svg"
+          alt="No journey data"
+          className="w-52 h-auto mb-2"
+        />
         <h4 className="font-semibold text-neutral-900 dark:text-white">
           No journey data yet
         </h4>

components/sites/SiteList.tsx

@@ -135,7 +135,15 @@ export default function SiteList({ sites, siteStats, loading, onDelete }: SiteLi
 
   if (sites.length === 0) {
     return (
-      <div className="rounded-lg border border-dashed border-neutral-300 dark:border-neutral-700 p-12 text-center">
+      <div className="rounded-2xl border border-dashed border-neutral-300 dark:border-neutral-700 p-12 text-center flex flex-col items-center">
+        <Image
+          src="/illustrations/setup-analytics.svg"
+          alt="Set up your first site"
+          width={280}
+          height={210}
+          className="mb-6"
+          unoptimized
+        />
         <h3 className="text-lg font-semibold text-neutral-900 dark:text-white">No sites yet</h3>
         <p className="mt-2 text-sm text-neutral-500 dark:text-neutral-400 mb-4">Create your first site to get started.</p>
         <Link href="/sites/new">