[feat](trx-rs): add build/install script and systemd user services

Provide a one-shot installer and matching systemd *user* units so the
server and client can be built, installed system-wide, and run in the
background without hand-rolled steps.

- script/install.sh: builds all three binaries (trx-server, trx-client,
  trx-configurator) in release mode and installs them to /usr/local/bin
  (configurable via --prefix/--bindir/PREFIX, sudo only when the target
  is not writable). Seeds ~/.config/trx-rs/trx-rs.toml from the example
  without ever overwriting existing config, then installs the user units
  with @BINDIR@ substituted to the real path and runs daemon-reload.
  Flags: --no-sdr, --no-build, --no-systemd, --enable-now.
- script/uninstall.sh: stops/disables the units, removes them and the
  binaries; keeps config unless --purge.
- packaging/systemd/{trx-server,trx-client}.service: user units reading
  the combined config at ~/.config/trx-rs/trx-rs.toml. The client softly
  depends on the server (Wants/After). KillSignal=SIGINT matches how the
  binaries shut down cleanly (SIGTERM is not handled).
- README: new "Install (optional, Linux + systemd)" section.

The web UI assets are embedded in the binary, so an install needs only
the binaries plus a config file — no data directory to ship.

Signed-off-by: Stan Grams <sjg@haxx.space>

packaging/systemd/trx-client.service

@@ -0,0 +1,37 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# systemd *user* unit for the trx-rs radio client and web frontend.
+#
+# Install with `script/install.sh` (which substitutes @BINDIR@ and copies this
+# into ~/.config/systemd/user/), or by hand:
+#   sed 's|@BINDIR@|/usr/local/bin|' trx-client.service \
+#     > ~/.config/systemd/user/trx-client.service
+#   systemctl --user daemon-reload
+#   systemctl --user enable --now trx-client.service
+#
+# Reads the combined config at ~/.config/trx-rs/trx-rs.toml ([trx-client]
+# section) and connects to the server (default 127.0.0.1:4530). The web UI
+# defaults to http://127.0.0.1:8080.
+
+[Unit]
+Description=trx-rs radio client and web frontend
+Documentation=https://github.com/stanislawgrams/trx-rs
+# Prefer the server to be up first; it is a soft dependency so the client still
+# starts (and retries) if the server is managed separately.
+Wants=trx-server.service
+After=trx-server.service
+
+[Service]
+Type=simple
+ExecStart=@BINDIR@/trx-client --config %h/.config/trx-rs/trx-rs.toml
+Restart=on-failure
+RestartSec=2
+# Shuts down cleanly on SIGINT (see trx-server.service for the rationale).
+KillSignal=SIGINT
+TimeoutStopSec=15
+NoNewPrivileges=true
+
+[Install]
+WantedBy=default.target

packaging/systemd/trx-server.service

@@ -0,0 +1,34 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# systemd *user* unit for the trx-rs radio server.
+#
+# Install with `script/install.sh` (which substitutes @BINDIR@ and copies this
+# into ~/.config/systemd/user/), or by hand:
+#   sed 's|@BINDIR@|/usr/local/bin|' trx-server.service \
+#     > ~/.config/systemd/user/trx-server.service
+#   systemctl --user daemon-reload
+#   systemctl --user enable --now trx-server.service
+#
+# Reads the combined config at ~/.config/trx-rs/trx-rs.toml ([trx-server]
+# section). Serial (/dev/ttyUSB*) and audio access require your user to be in
+# the `dialout` and `audio` groups — a user unit cannot grant them itself.
+
+[Unit]
+Description=trx-rs radio server (CAT/SDR backend)
+Documentation=https://github.com/stanislawgrams/trx-rs
+
+[Service]
+Type=simple
+ExecStart=@BINDIR@/trx-server --config %h/.config/trx-rs/trx-rs.toml
+Restart=on-failure
+RestartSec=2
+# Both binaries shut down cleanly on SIGINT (the server drains audio and exits
+# 0); systemd's default SIGTERM is not handled, so signal SIGINT instead.
+KillSignal=SIGINT
+TimeoutStopSec=15
+NoNewPrivileges=true
+
+[Install]
+WantedBy=default.target