Add -serve flag to serve the generated HTML over LAN

[aplgr]

Goal

Add a -serve flag that starts a tiny static file server so the generated HTML can be opened from other devices in the LAN.

Proposed behavior

• grovegrid -in data.csv -out out.html -serve generates as usual, then serves the output directory.
• Print URLs like:
  • http://localhost:PORT/
  • http://<LAN-IP>:PORT/

Flags

• -serve (bool)
• -host (default 0.0.0.0 so phones in LAN can reach it)
• -port (default 8080, or 0 for random free port)

Acceptance criteria

• Server is read-only static hosting.
• Logs one-line “Serving on …”.
• Ctrl+C cleanly stops.
• Doesn’t break the non-serve workflow.

Security note

This is intentionally minimal: LAN sharing only, no auth. Document that users should not expose it to the public internet.