Eliminate "Default" from thinking selector, always resolve to a real option

Like the model selector, the thinking selector now always resolves to an
actual thinking option ID instead of falling back to "Default" display text.

packages/app/src/hooks/use-agent-form-state.ts

@@ -161,7 +161,7 @@ function resolveThinkingOptionId(args: {
     return normalizedThinkingOptionId;
   }
 
-  return effectiveModel?.defaultThinkingOptionId ?? "";
+  return effectiveModel?.defaultThinkingOptionId ?? thinkingOptions[0]?.id ?? "";
 }
 
 /**