feat: add stop session button and update walkthrough

src/components/ActiveSessionBanner.tsx

@@ -2,7 +2,7 @@
 
 import React from 'react';
 import { useSession } from '@/context/SessionContext';
-import { GlassWater, X, ArrowRight, Sparkles } from 'lucide-react';
+import { GlassWater, Square, ArrowRight, Sparkles } from 'lucide-react';
 import Link from 'next/link';
 import { useI18n } from '@/i18n/I18nContext';
 
@@ -34,7 +34,7 @@ export default function ActiveSessionBanner() {
                     className="ml-4 p-2 hover:bg-white/10 rounded-full transition-colors"
                     title="End Session"
                 >
-                    <X size={20} />
+                    <Square size={20} fill="currentColor" />
                 </button>
             </div>
         </div>