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/tasks/main.yml

@@ -272,51 +272,55 @@
   notify: Restart sshd
 
 # ---------------------------------------------------------------------------
-# Phase 4 — VNC server (TigerVNC)
-# Persistent virtual display on :1 (port 5901). Replaces X11 forwarding.
+# Phase 4 — Xpra virtual display
+# Replaces TigerVNC. Rootless mode: each JMRI window appears as a native
+# window on the client. Sessions are persistent across disconnects.
+# Connect: xpra attach ssh://jmri@main-street-station/{{ jmri_xpra_display }}
 # ---------------------------------------------------------------------------
-- name: Install TigerVNC server
+- name: Remove TigerVNC (replaced by Xpra)
   ansible.builtin.apt:
     name: tigervnc-standalone-server
+    state: absent
+  notify: Reload systemd
+
+- name: Disable and stop jmri-vnc service if present
+  ansible.builtin.systemd:
+    name: jmri-vnc
+    enabled: false
+    state: stopped
+  failed_when: false
+
+- name: Remove jmri-vnc systemd unit if present
+  ansible.builtin.file:
+    path: /etc/systemd/system/jmri-vnc.service
+    state: absent
+  notify: Reload systemd
+
+- name: Remove jmri VNC password directory if present
+  ansible.builtin.file:
+    path: "{{ jmri_home }}/.vnc"
+    state: absent
+
+- name: Install Xpra
+  ansible.builtin.apt:
+    name: xpra
     state: present
     update_cache: false
 
-- name: Create .vnc directory for jmri user
-  ansible.builtin.file:
-    path: "{{ jmri_home }}/.vnc"
-    state: directory
-    owner: "{{ jmri_user }}"
-    group: "{{ jmri_group }}"
-    mode: '0700'
-
-- name: Set VNC password for jmri user
-  ansible.builtin.shell:
-    cmd: "printf '{{ jmri_vnc_password }}' | vncpasswd -f > {{ jmri_home }}/.vnc/passwd"
-  become_user: "{{ jmri_user }}"
-  no_log: true
-  changed_when: true
-
-- name: Set correct permissions on VNC password file
-  ansible.builtin.file:
-    path: "{{ jmri_home }}/.vnc/passwd"
-    owner: "{{ jmri_user }}"
-    group: "{{ jmri_group }}"
-    mode: '0600'
-
-- name: Deploy jmri-vnc systemd unit
+- name: Deploy jmri-xpra systemd unit
   ansible.builtin.template:
-    src: jmri-vnc.service.j2
-    dest: /etc/systemd/system/jmri-vnc.service
+    src: jmri-xpra.service.j2
+    dest: /etc/systemd/system/jmri-xpra.service
     owner: root
     group: root
     mode: '0644'
   notify:
     - Reload systemd
-    - Restart jmri-vnc
+    - Restart jmri-xpra
 
-- name: Enable and start jmri-vnc service
+- name: Enable and start jmri-xpra service
   ansible.builtin.systemd:
-    name: jmri-vnc
+    name: jmri-xpra
     enabled: true
     state: started
     daemon_reload: true