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/env.j2

@@ -60,4 +60,9 @@ NEXTCLOUD_URL=                  "{{ NEXTCLOUD_URL }}"
 JWT_SECRET_KEY=                 "{{ NEXTCLOUD_WHITEBOARD_JWT }}"
 STORAGE_STRATEGY=redis
 REDIS_URL=redis://redis:6379/0
+# Chromium (headless) hardening for Whiteboard
+CHROMIUM_FLAGS=--headless=new --no-sandbox --disable-gpu --disable-dev-shm-usage --use-gl=swiftshader --disable-software-rasterizer
+# Falls das Image Chromium mitbringt – Pfad meistens /usr/bin/chromium oder /usr/bin/chromium-browser:
+PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium
+PUPPETEER_SKIP_DOWNLOAD=true
 {% endif %}