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/config/main.yml

@@ -28,6 +28,8 @@ server:
 docker:
   volumes:
     data:                   nextcloud_data
+    whiteboard_tmp:         nextcloud_whiteboard_tmp
+    whiteboard_fontcache:   nextcloud_whiteboard_fontcache
   services:
     redis:
       enabled:              true