Web UI has no service worker, no font preload, and missing font-display: swap

[BYK]

Describe the bug

The web UI lacks three front-end performance optimizations:

1. No service worker — Every page load and refresh fetches all assets from the server. A service worker with appropriate caching strategies would make repeat visits near-instant.
2. No font preload — The JetBrainsMono font is only discovered after CSS evaluation, causing a late download. A preload link in the HTML head would start the download during HTML parse.
3. Missing font-display: swap — The @font-face declaration lacks font-display: swap, which can cause invisible text (FOIT) while the font loads.

Steps to reproduce

1. Open the web UI on a slow connection
2. Observe the font flash and slow initial load
3. Refresh the page — assets are re-fetched from the server

Expected behavior

• Service worker caches assets for instant repeat visits
• Font preload starts download early
• font-display: swap prevents invisible text

System info

• OpenCode version: v1.15.3
• OS: Linux

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• PWA does full app redownload on reload #19119: Same root cause — missing service worker causes full asset redownload on every reload
• [FEATURE]: Adding basic PWA support #19174: Feature request for basic PWA support including service worker

If your issue is distinct from these (e.g., you are specifically focused on the font preload and font-display: swap aspects), please clarify what's not covered by the existing issues.

[BYK]

Thanks — #19119 and #19174 cover the same ground. PR #27936 adds a service worker with appropriate caching strategies, plus font preload and font-display: swap.