Shrink AgentAvatar to sm + dim on idle instead of removing

Persistent presence is less jumpy than pop-in/out. When the model isn't
actively doing anything, the avatar collapses to the `sm` size variant,
loses its activity label, and dims to 50% opacity. Springs back to
full-size + label when activity resumes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/ai.tsx

@@ -571,14 +571,19 @@ function ChatSurface({
                 pendingConfirm: !!pendingConfirm,
                 confirmBusy,
               })
-              if (activity === "idle") return null
+              const isIdle = activity === "idle"
               return (
-                <div className="self-start">
+                <div
+                  className={`self-start transition-opacity duration-300 ${
+                    isIdle ? "opacity-50" : "opacity-100"
+                  }`}
+                >
                   <AgentAvatar
                     name={activeAgent?.name ?? "Atlas"}
                     activity={activity}
                     initials={activeAgent ? agentInitials(activeAgent.name) : "AT"}
-                    showLabel
+                    size={isIdle ? "sm" : "md"}
+                    showLabel={!isIdle}
                   />
                 </div>
               )