feat: add stage 0 foundation

frontend/src/components/app/AppPageHeader.tsx

@@ -0,0 +1,19 @@
+import type * as React from 'react'
+
+type AppPageHeaderProps = {
+  title: string
+  description?: string
+  actions?: React.ReactNode
+}
+
+export function AppPageHeader({ title, description, actions }: AppPageHeaderProps) {
+  return (
+    <div className="flex items-start justify-between gap-4">
+      <div>
+        <h1 className="text-xl font-semibold tracking-normal text-foreground">{title}</h1>
+        {description ? <p className="mt-1 text-sm text-muted-foreground">{description}</p> : null}
+      </div>
+      {actions ? <div className="flex items-center gap-2">{actions}</div> : null}
+    </div>
+  )
+}