Recover from half-done installs and missing devices instead of failing for good

- pi_shairport: on trixie systemd.pc moved into systemd-dev, so configure found no
  unit directory and `make install` died on an empty `install -d` - after it had
  already installed the binary. `creates:` then saw the binary and never retried, and
  the next tasks failed on the missing unit. Install systemd-dev on trixie, and key
  `creates:` on the unit file.
- pi_irserver: with the IRTrans receiver unplugged at boot, irserver exited at once and
  stayed failed. Restart it every 10s with no start limit until the device shows up.
- pi_modern_shell_env: get_url sent a conditional request to the oh-my-posh CDN on every
  run, so a TLS handshake timeout there failed the whole play with the binary already
  installed. Only download it when it is missing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/pi_irserver/files/irserver.service

@@ -1,10 +1,17 @@
 [Unit]
 Description=IR server for IR remotes
 After=network.target
+# Keep retrying rather than giving up: the IRTrans receiver is a USB serial device, and
+# when it is missing at boot (unplugged, or enumerated late) irserver exits at once with
+# "Error opening COM/USB Port". Without a restart it then stays failed until someone
+# notices the remote does nothing.
+StartLimitIntervalSec=0
 
 [Service]
 Type=simple
 ExecStart=/usr/bin/irserver /dev/ttyUSB0
+Restart=on-failure
+RestartSec=10
 
 [Install]
-WantedBy=multi-user.target
+WantedBy=multi-user.target