refactor: externalize AppPage component and streamline all tool pages

app/(app)/pastel/harmony/page.tsx

@@ -13,6 +13,7 @@ import {
   SelectValue,
 } from '@/components/ui/select';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
+import { AppPage } from '@/components/layout/AppPage';
 import { useGeneratePalette } from '@/lib/pastel/api/queries';
 import { Loader2, Palette } from 'lucide-react';
 import { toast } from 'sonner';
@@ -59,16 +60,11 @@ export default function HarmonyPage() {
   };
 
   return (
-    <div className="min-h-screen py-12">
-      <div className="max-w-7xl mx-auto px-8 space-y-8">
-        <div>
-          <h1 className="text-4xl font-bold mb-2">Harmony Palette Generator</h1>
-          <p className="text-muted-foreground">
-            Create color harmonies based on color theory principles
-          </p>
-        </div>
-
-        <div className="grid grid-cols-1 lg:grid-cols-2 gap-8">
+    <AppPage
+      title="Harmony Palette Generator"
+      description="Create color harmonies based on color theory principles"
+    >
+      <div className="grid grid-cols-1 lg:grid-cols-2 gap-8">
           {/* Controls */}
           <div className="space-y-6">
             <Card>
@@ -160,7 +156,6 @@ export default function HarmonyPage() {
             )}
           </div>
         </div>
-      </div>
-    </div>
+    </AppPage>
   );
 }