fix: beacon-transport login_start so the event survives OAuth navigation

src/analytics/ga4.ts

@@ -41,7 +41,7 @@ export {
 export type GA4EventName = 'login_start' | 'login' | 'select_content';
 
 export interface GA4EventProps {
-  login_start: { method: 'github' };
+  login_start: { method: 'github'; transport_type: 'beacon' };
   login: { method: 'github' };
   select_content: { content_type: 'product_cta'; item_id: string };
 }

src/analytics/ga4Model.mjs

@@ -29,8 +29,11 @@
 export const GA4_EVENT_PROPERTIES = {
   // Google recommended `login` event — method only, never any identity.
   login: { method: ['github'] },
-  // Funnel step: the GitHub OAuth button was activated.
-  login_start: { method: ['github'] },
+  // Funnel step: the GitHub OAuth button was activated. transport_type is
+  // a gtag control field (consumed by the tag, never sent as an ep param)
+  // — beacon sends immediately instead of batching, so the event survives
+  // the OAuth navigation that follows the click.
+  login_start: { method: ['github'], transport_type: ['beacon'] },
   // Google recommended `select_content` — used for the blog/product CTA.
   // item_id carries a slug-like identifier only (validated below).
   select_content: { content_type: ['product_cta'], item_id: null },

src/ledger/shared.tsx

@@ -43,7 +43,7 @@ export function GithubButton({ large = false, onLogin, remember = false }: { lar
     // pending flag lets the post-callback boot emit login_succeeded once.
     markLoginPending();
     captureEvent('github_login_started', {});
-    captureGoogleEvent('login_start', { method: 'github' });
+    captureGoogleEvent('login_start', { method: 'github', transport_type: 'beacon' });
     window.location.assign(remember ? '/api/auth/login?remember=1' : '/api/auth/login');
   };
   return (

test/ga4-analytics.test.mjs

@@ -177,7 +177,13 @@ test('GA4 event set is exactly the acquisition funnel', () => {
 
 test('login events carry method:github only — never identity', () => {
   assert.deepEqual(validateGA4Event('login', { method: 'github' }), { method: 'github' });
-  assert.deepEqual(validateGA4Event('login_start', { method: 'github' }), { method: 'github' });
+  assert.deepEqual(
+    validateGA4Event('login_start', { method: 'github', transport_type: 'beacon' }),
+    { method: 'github', transport_type: 'beacon' },
+  );
+  // beacon is required — a missing it falls back to batched sends that lose
+  // the event to the OAuth navigation
+  assert.equal(validateGA4Event('login_start', { method: 'github' }), null);
   assert.equal(validateGA4Event('login', { method: 'github', user_id: 'x' }), null);
   assert.equal(validateGA4Event('login', { method: 'github', email: 'a@b.c' }), null);
   assert.equal(validateGA4Event('login', { method: 'google' }), null);
@@ -266,7 +272,7 @@ test('GA4 wiring: boot init + pageview, popstate, navigate, login events', () =>
   assert.ok((pages.match(/captureGooglePageview\(\)/g) || []).length >= 2,
     'navigate + popstate must each emit a GA4 pageview');
   const shared = src('src/ledger/shared.tsx');
-  assert.match(shared, /captureGoogleEvent\('login_start', \{ method: 'github' \}\)/);
+  assert.match(shared, /captureGoogleEvent\('login_start', \{ method: 'github', transport_type: 'beacon' \}\)/);
 });
 
 test('GA4 and PostHog are separate modules — no cross-import', () => {