chore: strict Subdomain Validation with Detailed Error Messages

frontend/src/components/RelayForm.tsx

@@ -1,6 +1,7 @@
 import { createEffect, createMemo, createSignal, For } from "solid-js"
 import type { Relay } from "@/lib/hooks"
 import { slugify } from "@/lib/slugify"
+import { validateSubdomainLabel } from "@/lib/subdomain"
 import { setToastMessage } from "@/components/Toast"
 import { plans } from "@/lib/state"
 
@@ -31,6 +32,12 @@ export default function RelayForm(props: RelayFormProps) {
       return
     }
 
+    const subdomainError = validateSubdomainLabel(subdomain())
+    if (subdomainError) {
+      setToastMessage(subdomainError)
+      return
+    }
+
     setToastMessage("")
     setSubmitting(true)