feat: add PageHeader component and centralize page spacing

- Create reusable PageHeader component (emoji + title + subtitle + actions)
- Use PageHeader in sessions, teams, users, objectives pages
- Centralize vertical padding in layout (py-6) and remove per-page py-* values

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/weather/new/page.tsx

@@ -63,7 +63,7 @@ export default function NewWeatherPage() {
   }
 
   return (
-    <main className="mx-auto max-w-2xl px-4 py-8">
+    <main className="mx-auto max-w-2xl px-4">
       <Card>
         <CardHeader>
           <CardTitle className="flex items-center gap-2">