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/docker-compose.yml.j2

@@ -70,6 +70,10 @@
 {% include 'roles/docker-container/templates/healthcheck/nc.yml.j2' %}
     image: "{{ NEXTCLOUD_WHITEBOARD_IMAGE }}:{{ NEXTCLOUD_WHITEBOARD_VERSION }}"
     container_name: {{ NEXTCLOUD_WHITEBOARD_CONTAINER }}
+    volumes:
+      - whiteboard_tmp:/tmp
+      - whiteboard_fontcache:/var/cache/fontconfig
+
     expose:
       - "{{ container_port }}"
     networks:
@@ -77,6 +81,7 @@
         ipv4_address: 192.168.102.71
 {% endif %}
 
+
 {% set service_name = NEXTCLOUD_CRON_SERVICE %}
   {{ service_name }}:
     container_name: "{{ NEXTCLOUD_CRON_CONTAINER }}"
@@ -99,5 +104,11 @@
 {% include 'roles/docker-compose/templates/volumes.yml.j2' %}
   data:
     name: {{ NEXTCLOUD_VOLUME }}
+{% if NEXTCLOUD_WHITEBOARD_ENABLED %}
+  whiteboard_tmp:
+    name: {{ NEXTCLOUD_WHITEBOARD_TMP_VOLUME }}
+  whiteboard_fontcache:
+    name: {{ NEXTCLOUD_WHITEBOARD_FRONTCACHE_VOLUME }}
+{% endif %}
 
 {% include 'roles/docker-compose/templates/networks.yml.j2' %}