Add service worker with Workbox: cache manifest, image cache, font cache, skip_waiting

- sw.js with network-first strategy, versioned cache, auto-purge old caches
- Workbox: cache_manifest, image_cache (30 days, 200 entries), font_cache (30 days, 10 entries)
- skip_waiting + clients.claim for instant updates
- CSP nonce for SW registration script
- Remove sw.js and workbox from .gitignore

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/base.html.twig

@@ -4,7 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{% block title %}{% endblock %}</title>
-    {{ pwa(injectThemeColor: true, injectFavicons: true, injectSW: false, injectResourceHints: true, injectSpeculationRules: true) }}
+    {{ pwa(injectThemeColor: true, injectFavicons: true, injectSW: true, swAttributes: {nonce: csp_nonce('script')}, injectResourceHints: true, injectSpeculationRules: true) }}
     {% block meta %}
         <meta name="description" content="{% block description %}E-Ticket - Plateforme de vente de tickets evenementiels pour associations{% endblock %}">
     {% endblock %}