perf(aiui): chat backdrop 1052K → 478K webp (was the 'ages to load' report)

The 2912×1632 jpg painted visibly slowly over Tailscale/Tor. 1920w q82
webp is visually identical behind glass and under half the weight.

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

aiui/packages/app/src/pages/ChatPage.vue

@@ -402,7 +402,9 @@ const { isDark } = useTheme()
 const isEmbedded = !!(window as unknown as Record<string, unknown>).__AIUI_EMBEDDED__
 
 // Background image URL — use BASE_URL for correct path in all deployments
-const bgImageUrl = `${import.meta.env.BASE_URL}assets/img/bg-intro-3.jpg`
+// 2912×1632 jpg was 1052K — visible as a slow paint over Tailscale/Tor.
+// The webp (1920w, q82) is 478K with no visible difference behind glass.
+const bgImageUrl = `${import.meta.env.BASE_URL}assets/img/bg-intro-3.webp`
 
 useAI()