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>
+  );
+}

components/layout/Navbar.tsx

@@ -19,14 +19,16 @@ const navItems = [
 export function Navbar() {
   const pathname = usePathname();
   const [mounted, setMounted] = useState(false);
-  const { theme, setTheme, resolvedTheme } = useTheme();
+  const themeContext = useTheme();
 
   useEffect(() => {
     setMounted(true);
   }, []);
 
   const toggleTheme = () => {
-    setTheme(resolvedTheme === 'dark' ? 'light' : 'dark');
+    if (themeContext) {
+      themeContext.setTheme(themeContext.resolvedTheme === 'dark' ? 'light' : 'dark');
+    }
   };
 
   return (
@@ -59,14 +61,14 @@ export function Navbar() {
         </div>
 
         {/* Theme Toggle */}
-        {mounted && (
+        {mounted && themeContext && (
           <Button
             variant="ghost"
             size="icon"
             onClick={toggleTheme}
             aria-label="Toggle theme"
           >
-            {resolvedTheme === 'dark' ? (
+            {themeContext.resolvedTheme === 'dark' ? (
               <Sun className="h-5 w-5" />
             ) : (
               <Moon className="h-5 w-5" />