[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>

README.md

@@ -116,6 +116,39 @@ section and the client reads `[trx-client]`.
 
 Open the configured HTTP frontend address in a browser (default `http://localhost:8080`).
 
+### 5. Install (optional, Linux + systemd)
+
+To build, install the binaries system-wide, and set up systemd **user** services
+that run the server and client in the background:
+
+```bash
+script/install.sh
+```
+
+This builds in release mode, installs `trx-server`, `trx-client`, and
+`trx-configurator` to `/usr/local/bin` (using `sudo` only if needed), seeds
+`~/.config/trx-rs/trx-rs.toml` from the example (existing config is never
+overwritten), and installs `~/.config/systemd/user/{trx-server,trx-client}.service`.
+
+```bash
+script/install.sh --prefix ~/.local   # install to ~/.local/bin instead
+script/install.sh --no-sdr            # build without SoapySDR support
+script/install.sh --enable-now        # also enable + start the services now
+script/install.sh --help              # all options
+```
+
+After editing your config, manage the services with:
+
+```bash
+systemctl --user enable --now trx-server trx-client   # start now + on login
+journalctl --user -u trx-server -u trx-client -f      # follow logs
+loginctl enable-linger "$USER"                        # keep running after logout
+```
+
+Serial (`/dev/ttyUSB*`) and audio access require your user to be in the
+`dialout` and `audio` groups. Remove everything with `script/uninstall.sh`
+(add `--purge` to also delete the config).
+
 ## How It Works
 
 ```mermaid