feat(HAI-002): build header and toast components

packages/dashboard/app/components/Header.tsx

@@ -0,0 +1,17 @@
+interface HeaderProps {
+  onNewTask: () => void;
+}
+
+export function Header({ onNewTask }: HeaderProps) {
+  return (
+    <header className="header">
+      <div className="header-left">
+        <h1 className="logo">hai</h1>
+        <span className="logo-sub">board</span>
+      </div>
+      <button className="btn btn-primary" onClick={onNewTask}>
+        + New Task
+      </button>
+    </header>
+  );
+}