V2 web UI returns 404 for empty assets, breaking service worker installation

Description

The V2 web UI service worker fails installation while precaching an empty JavaScript asset:

bad-precaching-response :: [{"url":"https://<server>/_assets/__vite-browser-external-2447137e-BvRk9kiK.js","status":404}]

The worker transitions to redundant. Its precache entry has the SHA-256 integrity of an empty file. In packages/cli/src/services/web-ui.ts, serveUI uses if (!file) after looking up the asset; valid empty text assets therefore return 404 instead of an empty 200 response.

Plugins

Not applicable to static asset serving.

OpenCode version

0.0.0-beta-19151; the falsy asset check is also present on v2 at 7ad705225e52160e9956fb727ac7c0801b3ac979.

Steps to reproduce

1. Open the V2 served web UI in a fresh Chromium profile.
2. Inspect the service worker registration and installation errors (or register /sw.js manually).
3. Observe the failed precache request above and the worker becoming redundant.
4. A minimal server regression fixture is an AssetMap containing "_assets/empty.js": "": GET and HEAD currently return 404, though the asset exists.

Screenshot and/or share link

The diagnostic above is from Chromium's ServiceWorker.workerErrorReported event. No session data is needed to reproduce.

Operating System

Arch Linux, Chromium.

Terminal

Not applicable.