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/components/ui/PageHeader.tsx

@@ -0,0 +1,24 @@
+import { ReactNode } from 'react';
+
+interface PageHeaderProps {
+  emoji: string;
+  title: string;
+  subtitle?: ReactNode;
+  actions?: ReactNode;
+  className?: string;
+}
+
+export function PageHeader({ emoji, title, subtitle, actions, className }: PageHeaderProps) {
+  return (
+    <div className={`mb-8 flex flex-col sm:flex-row sm:items-center justify-between gap-4 ${className ?? ''}`}>
+      <div>
+        <h1 className="flex items-center gap-3 text-3xl font-bold tracking-tight text-foreground">
+          <span className="text-3xl leading-none">{emoji}</span>
+          {title}
+        </h1>
+        {subtitle && <p className="mt-1.5 text-sm text-muted">{subtitle}</p>}
+      </div>
+      {actions && <div className="shrink-0">{actions}</div>}
+    </div>
+  );
+}