Package the app/daemon as a .deb #15

Summary

Build and distribute the app + gateway daemon as a Debian package (.deb) so the Raspberry Pi (and other Debian/Ubuntu hosts) can install/upgrade it cleanly instead of the current rsync + manual rebuild + service-setup flow.

Motivation

The Pi is currently deployed by rsync from the laptop, then a manual rebuild and a hand-rolled systemd setup via gateway/install.sh (see README). A .deb gives versioned install/upgrade/remove, dependency declaration, and proper service registration through the standard package manager — and pairs naturally with the in-app updater (#13).

What the package should do

• Install the built GUI (dist/) and the gateway daemon.
• Install/enable the plotter-gateway systemd service (replacing the manual setup in gateway/install.sh).
• Declare runtime dependencies (Node, serial access / dialout group, udev rule for a stable device path).
• Apply the always-on power profile and mDNS bits currently handled by the installer, where appropriate.
• Carry a real version so install/upgrade/remove behave correctly.

Acceptance criteria

• A repeatable build produces a versioned .deb.
• Installing the .deb on the Pi sets up the GUI, daemon, and systemd service.
• Dependencies (Node, serial access, udev rule) are declared/handled by the package.
• Upgrading via the package preserves persisted state (plotter state, settings).
• Removing the package cleans up the service.

Notes

• Fold in the logic from gateway/install.sh (service unit, dialout membership, udev rule, power profile, mDNS).
• The native serialport binding must build/ship correctly for the Pi architecture (arm).
• Pairs with the updater (In-app updater (local machine and Raspberry Pi) #13) — the updater can drive package upgrades instead of rsync/rebuild.
• ARM target (Raspberry Pi) is primary; document/support amd64 if the laptop should also install it.