fix: move themeColor to viewport export (Next.js 16)

Move themeColor from metadata export to viewport export as required
by Next.js 16 to fix the unsupported metadata warning.

Co-Authored-By: Claude (glm-4.7) <noreply@anthropic.com>

src/app/layout.tsx

@@ -1,4 +1,4 @@
-import type { Metadata } from "next";
+import type { Metadata, Viewport } from "next";
 import localFont from "next/font/local";
 import "./globals.css";
 import "./fonts.css";
@@ -12,12 +12,14 @@ const inter = localFont({
   display: "swap",
 });
 
+export const viewport: Viewport = {
+  themeColor: "#64748B",
+};
+
 export const metadata: Metadata = {
   title: "Brachnha Insight",
   description: "Transform what in your mind into a powerful notes",
-  // Story 3.4: PWA metadata
-  manifest: "/manifest.webmanifest", // Next.js 14+ convention
-  themeColor: "#64748B",
+  manifest: "/manifest.webmanifest",
   appleWebApp: {
     capable: true,
     statusBarStyle: "default",