perf: add Link preload headers for critical boot assets
Browser starts fetching boot.mjs and style.css while still parsing
the HTML response, saving ~1 RTT on first load. Combined with
Cloudflare edge caching (now enabled for .mjs files via cache rule),
users worldwide get cached assets from their nearest Cloudflare POP
instead of hitting SFO3 origin every time.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>