Serve HTML over HTTP in non-watch mode

[scott]

Replace the file:// URL with a localhost HTTP server even when --watch is not set. Benefits:

• fetch()/XHR from the rendered page works (currently blocked by file:// origin null)
• Consistent behavior between watch and non-watch modes
• Easier to share a preview on the LAN

Tradeoffs:

• Process must stay running (breaks the current fire-and-forget UX in main)
• Need to pick a port (default to OS-assigned)
• Need cleanup on exit

Tracked from the live-reload implementation PR.