Fix WCAG contrast issues: yellow-500->700, indigo-600->800 on links

- text-yellow-500 on white bg had ratio ~1.9 (need 4.5), now text-yellow-700
- text-indigo-600 links on white bg had ratio ~3.8, now text-indigo-800
  with permanent underline for link visibility (WCAG 1.4.1)
- Cookie banner link also updated

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

templates/base.html.twig

@@ -190,7 +190,7 @@
     <div id="cookie-banner" class="hidden fixed bottom-0 left-0 right-0 z-50 border-t-4 border-gray-900 bg-white p-4">
         <div class="max-w-4xl mx-auto flex flex-col sm:flex-row items-center justify-between gap-4">
             <p class="text-sm font-bold text-gray-700">
-                Ce site utilise des cookies pour mesurer l'audience. <a href="{{ path('app_cookies') }}" class="text-indigo-600 hover:underline">En savoir plus</a>
+                Ce site utilise des cookies pour mesurer l'audience. <a href="{{ path('app_cookies') }}" class="text-indigo-800 underline hover:text-indigo-950">En savoir plus</a>
             </p>
             <div class="flex gap-2">
                 <button id="cookie-refuse" class="px-4 py-2 border-2 border-gray-900 bg-white font-black uppercase text-xs tracking-widest hover:bg-gray-100 transition-all cursor-pointer">Refuser</button>