fix: add error handling for agent env vars save and sprites-exec JSON errors

- Agent env vars: auto-save now awaits response and shows alert on failure
- sprites-exec v0.1.4: detect JSON error responses (e.g., auth failures) before binary parsing

apps/web/src/app/dashboard/agents/[id]/page.tsx

@@ -1029,14 +1029,26 @@ export default function AgentDetailPage(): React.ReactElement {
         <div className="bg-surface border border-border rounded-lg p-6">
           <EnvVarsEditor
             value={envVars}
-            onChange={(newEnvVars) => {
+            onChange={async (newEnvVars) => {
               setEnvVars(newEnvVars);
-              // Auto-save env vars
-              fetch(`/api/agents/${agentId}`, {
-                method: 'PATCH',
-                headers: { 'Content-Type': 'application/json' },
-                body: JSON.stringify({ envVars: newEnvVars }),
-              });
+              // Auto-save env vars with error handling
+              try {
+                const response = await fetch(`/api/agents/${agentId}`, {
+                  method: 'PATCH',
+                  headers: { 'Content-Type': 'application/json' },
+                  body: JSON.stringify({ envVars: newEnvVars }),
+                });
+                const result = await response.json();
+                if (!result.success) {
+                  console.error('Failed to save env vars:', result.error);
+                  alert(
+                    'Failed to save environment variables: ' + (result.error || 'Unknown error')
+                  );
+                }
+              } catch (err) {
+                console.error('Failed to save env vars:', err);
+                alert('Failed to save environment variables. Please try again.');
+              }
             }}
             title="Environment Variables"
             description="Passed to tools at runtime. Agent vars override collection vars. Changes are saved automatically."