jmri: replace TigerVNC with Xpra for JMRI GUI display

VNC had window management issues and some dialogs wouldn't open correctly.
Xpra runs in rootless mode — each JMRI window appears natively on the
client without a VNC client or XQuartz required.

Changes:
- Remove tigervnc-standalone-server, jmri-vnc.service, .vnc/ directory
- Install xpra, deploy jmri-xpra.service.j2 (systemd unit)
- Update jmri-gui wrapper: DISPLAY=:100, attach instructions printed on launch
- Update defaults: drop VNC vars, add jmri_xpra_display=100
- Rename handler: Restart jmri-vnc -> Restart jmri-xpra

Connect from macOS/Linux:
  xpra attach ssh://jmri@main-street-station/100

ansible/roles/jmri/templates/jmri-xpra.service.j2

@@ -0,0 +1,26 @@
+[Unit]
+Description=Xpra virtual display for JMRI (:{{ jmri_xpra_display }})
+After=network.target
+# Start before jmri.service so the display is ready when JMRI launches
+Before=jmri.service
+
+[Service]
+Type=simple
+User={{ jmri_user }}
+Group={{ jmri_group }}
+ExecStart=/usr/bin/xpra start \
+    :{{ jmri_xpra_display }} \
+    --daemon=no \
+    --mdns=no \
+    --notifications=no \
+    --systemd-run=no \
+    --pulseaudio=no \
+    --speaker=off \
+    --microphone=off \
+    --video-encoders=none \
+    --start-via-proxy=no
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target