In-app updater (local machine and Raspberry Pi)

[BernardJen]

Summary

Add an updater so the app/gateway can be updated to a new version without a manual rebuild — covering both the local machine and the Raspberry Pi deployment.

Motivation

Today the Pi is updated by rsync + manual rebuild + service restart from the laptop (see README "Deploying updates"), and there is no in-app notion of "a new version is available." An updater makes upgrades discoverable and low-effort, especially for the headless Pi.

Behavior

• Detect that a newer version is available (e.g. compare against a release/tag or remote source).
• Show the current version and the available version in the UI.
• Trigger an update: fetch the new code, rebuild the GUI, and restart the gateway service.
• Report progress and success/failure; do not interrupt a running plot — defer the restart until the plotter is idle.

Targets

• Raspberry Pi — update the daemon + GUI in place and restart the plotter-gateway systemd service.
• Local machine — at minimum surface current vs. available version and the steps/command to update.

Acceptance criteria

• App shows the current version.
• App detects when a newer version is available.
• User can trigger an update (at least on the Pi) from the UI.
• Update fetches, rebuilds, and restarts the service.
• Progress and outcome are reported; a running plot is never interrupted.

Notes

• Pi update path should mirror the existing rsync/rebuild/restart flow in the README and gateway/install.sh.
• Needs new gateway endpoint(s) to drive the update (gateway/server.ts).
• Define how "latest version" is determined (git tag / release / commit).

[BernardJen]

Pairs with #15 (.deb packaging) — once packaged, the updater can drive apt/package upgrades on the Pi instead of rsync + manual rebuild.