CORS/CSP hardening & centralization

- Add reusable Nginx include: roles/sys-svc-proxy/templates/headers/access_control_allow.conf.j2
  (dynamic ACAO/credentials/methods/headers via role vars)
- Set global 'Vary: Origin' in nginx.conf.j2 to prevent cache poisoning
- CSP: allow Simple Icons via connect-src when feature is enabled
- Front proxy: rename vars to lowercase + flush handlers after config deploy
- Desktop: gate & load Simple Icons role; inject brand logos when enabled
- Bluesky + Logout: replace inline CORS with centralized include
- Simpleicons: public CORS (ACAO='*', no credentials), keep GET/OPTIONS, allow headers
- Taiga: adjust canonical domain to taiga.kanban.{{ PRIMARY_DOMAIN }}
- LibreTranslate: remove unused images/versions keys

Fixes: https://open.project.infinito.nexus/projects/cymais/work_packages/342/activity
Discussion: https://chatgpt.com/share/68da5e27-ffd4-800f-91a3-0ef103058d44

roles/sys-svc-webserver-core/templates/nginx.conf.j2

@@ -13,6 +13,11 @@ http
 
   default_type text/html;
 
+  {# Ensure caches (browsers, proxies, CDNs) treat responses as dependent on the Origin header
+  to prevent cross-domain cache poisoning issues.
+  Discussion: https://chat.openai.com/share/2671b961-c1b0-472d-bae2-2804d0455e8a #}
+  add_header 'Vary' 'Origin' always;
+
   {# caching #}
   proxy_cache_path {{ NGINX.DIRECTORIES.CACHE.GENERAL }} levels=1:2 keys_zone=cache:20m max_size=20g inactive=14d use_temp_path=off;
   proxy_cache_path {{ NGINX.DIRECTORIES.CACHE.IMAGE }} levels=1:2 keys_zone=imgcache:10m inactive=60m use_temp_path=off;