feat: add shared app layout with nav

Creates the <x-layouts.app> Blade component that provides the main
application shell, including Livewire styles/scripts, top navigation
linking to Sites and Social Accounts pages, and a slot for page content.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

resources/views/components/layouts/app.blade.php

@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="utf-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1">
+    <title>{{ $title ?? 'Muninn' }}</title>
+    @vite('resources/css/app.css')
+    @livewireStyles
+</head>
+<body class="bg-gray-50 text-gray-900">
+    <nav class="bg-white border-b border-gray-200 px-6 py-4 flex gap-6">
+        <a href="{{ route('sites.index') }}" class="font-medium hover:text-blue-600">Sites</a>
+        <a href="{{ route('social-accounts.index') }}" class="font-medium hover:text-blue-600">Social Accounts</a>
+    </nav>
+
+    <main class="p-6">
+        {{ $slot }}
+    </main>
+
+    @livewireScripts
+</body>
+</html>