feat(#417): add fetchWithRetry with exponential backoff for auth

Retries network and server errors up to 3 times with exponential
backoff (1s, 2s, 4s). Non-retryable errors fail immediately.

Refs #417

apps/web/src/lib/auth/sleep.ts

@@ -0,0 +1,9 @@
+/**
+ * Wait for the specified number of milliseconds.
+ *
+ * Extracted to a separate module to enable clean test mocking
+ * without fake timers.
+ */
+export function sleep(ms: number): Promise<void> {
+  return new Promise<void>((resolve) => setTimeout(resolve, ms));
+}