fix: resolve ThemeProvider SSR issue causing 500 error

The app was crashing with "useTheme must be used within ThemeProvider"
error during server-side rendering.

Changes:
- Created AppLayout client component to wrap Navbar
- Modified useTheme hook to return default values during SSR
  instead of throwing an error
- Updated Navbar to safely handle theme context
- Moved Navbar rendering into client-side only AppLayout

This fixes the SSR hydration mismatch and allows the app
to render correctly on both server and client.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/layout/AppLayout.tsx

@@ -0,0 +1,13 @@
+'use client';
+
+import { ReactNode } from 'react';
+import { Navbar } from './Navbar';
+
+export function AppLayout({ children }: { children: ReactNode }) {
+  return (
+    <div className="flex min-h-screen">
+      <Navbar />
+      <main className="flex-1 container mx-auto px-4 py-8">{children}</main>
+    </div>
+  );
+}