refactor: Update Header component props to make url required

Make the `url` prop in the Header component required by changing its type to
`string | null`. This change ensures that a URL is always provided for the
Header component to function correctly.

components/flow/Header.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 interface Props {
-	url?: string
+	url: string | null
 	title: string
 	id: string
 }