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/layout.tsx

@@ -46,7 +46,9 @@ export default function RootLayout({
         <Providers>
           <div className="min-h-screen bg-background">
             <Header />
-            {children}
+            <div className="py-6">
+              {children}
+            </div>
           </div>
         </Providers>
       </body>