Add subscription validation and first-run onboarding reveal

src/web/api.js

@@ -16,6 +16,11 @@ async function request(url, options = {}) {
 export const api = {
   state: () => request('/api/state'),
   subscription: {
+    validate: (url, signal) => request('/api/subscription/validate', {
+      method: 'POST',
+      body: JSON.stringify({ url }),
+      signal,
+    }),
     fetch: (url) => request('/api/subscription/fetch', {
       method: 'POST',
       body: JSON.stringify({ url }),