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/handlers/main.yml

@@ -17,9 +17,9 @@
     name: jmri
     state: restarted
 
-- name: Restart jmri-vnc
+- name: Restart jmri-xpra
   ansible.builtin.systemd:
-    name: jmri-vnc
+    name: jmri-xpra
     state: restarted
 
 - name: Restart sshd