fix(engine): widen AuthStorage cast in proxy set trap to satisfy tsc

TS2352: Direct conversion from AuthStorage to Record<string|symbol, unknown>
no longer overlaps. Route through `unknown` so the proxy set trap continues
to forward writes to the underlying target.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/auth-storage.ts

@@ -146,7 +146,7 @@ export function createFusionAuthStorage(): AuthStorage {
     // underlying AuthStorage. Without this trap, writes land on the Proxy
     // object itself and the target's fallbackResolver stays undefined.
     set(target: AuthStorage, prop: string | symbol, value: unknown) {
-      (target as Record<string | symbol, unknown>)[prop] = value;
+      (target as unknown as Record<string | symbol, unknown>)[prop] = value;
       return true;
     },