[fix](trx-rs): install binaries under user home

Default installation and removal to /home/sjg/.local/bin while preserving explicit prefix and binary-directory overrides. Update the service examples and README to match.

Assisted-By: OpenAI Codex (GPT-5)
Signed-off-by: Stan Grams <sjg@haxx.space>

packaging/systemd/trx-client.service

@@ -6,7 +6,7 @@
 #
 # 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 \
+#   sed "s|@BINDIR@|$HOME/.local/bin|" trx-client.service \
 #     > ~/.config/systemd/user/trx-client.service
 #   systemctl --user daemon-reload
 #   systemctl --user enable --now trx-client.service

packaging/systemd/trx-server.service

@@ -6,7 +6,7 @@
 #
 # 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 \
+#   sed "s|@BINDIR@|$HOME/.local/bin|" trx-server.service \
 #     > ~/.config/systemd/user/trx-server.service
 #   systemctl --user daemon-reload
 #   systemctl --user enable --now trx-server.service