Enhancement: Fix for Nextcloud Whiteboard recording and collaboration server

- Added Chromium headless flags and writable font cache/tmp volumes
- Enabled WebSocket proxy forwarding for /whiteboard/
- Verified and adjusted CSP and frontend integration
- Added Whiteboard-related variables and volumes in main.yml

See ChatGPT conversation (20 Oct 2025):
https://chatgpt.com/share/68f655e1-fa3c-800f-b35f-4f875dfed4fd

roles/web-app-nextcloud/templates/nginx/host.conf.j2

@@ -23,6 +23,12 @@ server
     {% include 'roles/sys-svc-proxy/templates/location/ws.conf.j2' %}
   {% endif %}
 
+  {% if NEXTCLOUD_WHITEBOARD_ENABLED | bool %}
+    {% set location_ws = '^~ ' ~ NEXTCLOUD_WHITEBOARD_LOCATION %}
+    {% set ws_port     = NEXTCLOUD_PORT %}
+    {% include 'roles/sys-svc-proxy/templates/location/ws.conf.j2' %}
+  {% endif %}
+
   {% include 'roles/sys-svc-proxy/templates/location/html.conf.j2' %}
 
   location ^~ /.well-known {