Improve empty encounter UX with interactive add button

Replace the static "No combatants yet" text with a centered, breathing
"+" icon that focuses the action bar input on click, guiding users to
add their first combatant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/index.css

@@ -80,6 +80,22 @@
 	}
 }
 
+@keyframes breathe {
+	0%,
+	100% {
+		opacity: 0.4;
+		scale: 0.9;
+	}
+	50% {
+		opacity: 1;
+		scale: 1.1;
+	}
+}
+
+@utility animate-breathe {
+	animation: breathe 3s ease-in-out infinite;
+}
+
 @custom-variant pointer-coarse (@media (pointer: coarse));
 
 @utility animate-confirm-pulse {