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/gif-mood/[id]/page.tsx

@@ -29,7 +29,7 @@ export default async function GifMoodSessionPage({ params }: GifMoodSessionPageP
   const userTeams = await getUserTeams(authSession.user.id);
 
   return (
-    <main className="mx-auto max-w-7xl px-4 py-8">
+    <main className="mx-auto max-w-7xl px-4">
       {/* Header */}
       <div className="mb-8">
         <div className="flex items-center gap-2 text-sm text-muted mb-2">