Harden iOS sign-in against stale-connection POST failures

iPad users hit a spurious "network error" on the first tap of
"Send sign-in link", with a second tap succeeding. Cause is iOS
Safari dispatching the POST onto a pooled keep-alive socket the
server/proxy already closed; unlike a GET it isn't transparently
re-sent, so it surfaces as a transport TypeError. The single 500ms
auto-retry was too quick and reused the same dead socket.

Both sign-in entry points (auth.html postWithRetry, index.html
fetchWithRetry) now retry 3x with growing backoff (0 -> +400ms ->
+1.6s) to outlast Safari evicting the socket. Frontend-only.

Ships as 0.2.156.

startos/versions/v0.2.156.ts

@@ -0,0 +1,12 @@
+import { VersionInfo } from '@start9labs/start-sdk'
+
+export const v_0_2_156 = VersionInfo.of({
+  version: '0.2.156:0',
+  releaseNotes: {
+    en_US: 'Sign-in is more reliable on iPad/iPhone: the "Send sign-in link" button now retries a few times with growing backoff when Safari dispatches the request onto a stale connection, so it no longer shows a spurious "network error" on the first tap.',
+  },
+  migrations: {
+    up: async ({ effects }) => {},
+    down: async ({ effects }) => {},
+  },
+})