🎮 Add landing page with hero selection (Story 1.5)

Landing page with animated tagline, dual CTA (adventure/express), and
HeroSelector component (3 heroes: Recruiter, Client, Developer) with full
keyboard accessibility (radiogroup, arrow nav, Enter confirm). Staggered
CSS animations respecting prefers-reduced-motion. Bilingual FR/EN.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/app/assets/css/transitions.css

@@ -37,6 +37,44 @@
   transform: translateY(-8px);
 }
 
+/* Landing page animations */
+@keyframes fadeSlideUp {
+  from {
+    opacity: 0;
+    transform: translateY(20px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+.animate-fade-slide-up {
+  opacity: 0;
+  animation: fadeSlideUp 0.6s ease-out forwards;
+}
+
+.animate-delay-100 { animation-delay: 0.1s; }
+.animate-delay-200 { animation-delay: 0.2s; }
+.animate-delay-300 { animation-delay: 0.3s; }
+.animate-delay-400 { animation-delay: 0.4s; }
+
+/* Hero selector transition */
+.hero-enter-active,
+.hero-leave-active {
+  transition: opacity 0.4s ease, transform 0.4s ease;
+}
+
+.hero-enter-from {
+  opacity: 0;
+  transform: translateY(20px);
+}
+
+.hero-leave-to {
+  opacity: 0;
+  transform: translateY(-20px);
+}
+
 /* Respect de prefers-reduced-motion */
 @media (prefers-reduced-motion: reduce) {
   .page-enter-active,
@@ -53,8 +91,21 @@
   .layout-enter-from,
   .layout-leave-to,
   .slide-down-enter-from,
-  .slide-down-leave-to {
+  .slide-down-leave-to,
+  .hero-enter-from,
+  .hero-leave-to {
     opacity: 1;
     transform: none;
   }
+
+  .animate-fade-slide-up {
+    animation: none;
+    opacity: 1;
+    transform: none;
+  }
+
+  .hero-enter-active,
+  .hero-leave-active {
+    transition: none;
+  }
 }