Unset app ID from builder store when unmounting to prevent stale app ID header

packages/builder/src/pages/builder/app/[application]/_layout.svelte

@@ -7,7 +7,7 @@
   import { API } from "api"
   import { isActive, goto, layout, redirect } from "@roxi/routify"
   import { capitalise } from "helpers"
-  import { onMount } from "svelte"
+  import { onMount, onDestroy } from "svelte"
 
   export let application
 
@@ -66,6 +66,13 @@
       hasSynced = true
     }
   })
+
+  onDestroy(() => {
+    store.update(state => {
+      state.appId = null
+      return state
+    })
+  })
 </script>
 
 {#await promise}