fix: dark background + emoji favicon for rStack design consistency

Standardize to dark background (#0f172a / slate-900) and add emoji
data URI favicon matching the rStack ecosystem.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/globals.css

@@ -3,8 +3,8 @@
 @tailwind utilities;
 
 :root {
-  --background: #0a0a0a;
-  --foreground: #ededed;
+  --background: #0f172a;
+  --foreground: #e2e8f0;
 }
 
 body {

src/app/layout.tsx

@@ -11,6 +11,9 @@ const inter = Inter({
 export const metadata: Metadata = {
   title: 'rTrips - Collaborative Trip Planning',
   description: 'Plan trips together with natural language input, structured itineraries, and a real-time collaborative canvas. Describe your trip and let AI structure it for you.',
+  icons: {
+    icon: "data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>✈️</text></svg>",
+  },
   openGraph: {
     title: 'rTrips - Collaborative Trip Planning',
     description: 'Plan trips together with natural language input and real-time collaborative canvas.',