refactor: externalize AppPage component and streamline all tool pages

app/(app)/units/page.tsx

@@ -1,17 +1,13 @@
 import MainConverter from '@/components/units/MainConverter';
+import { AppPage } from '@/components/layout/AppPage';
 
 export default function UnitsPage() {
   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">Units Converter</h1>
-          <p className="text-muted-foreground">
-            Smart unit converter with 187 units across 23 categories
-          </p>
-        </div>
-        <MainConverter />
-      </div>
-    </div>
+    <AppPage 
+      title="Units Converter" 
+      description="Smart unit converter with 187 units across 23 categories"
+    >
+      <MainConverter />
+    </AppPage>
   );
 }