fix: prevent Getting Started hint flash and Playground provider ID flash

- Getting Started hint now waits for workflowsLoaded before rendering
- Playground provider selector shows placeholder until providers load

Author: idemerge

frontend/src/App.tsx

@@ -465,7 +465,7 @@ function App() {
                   className="space-y-6"
                 >
                   {/* Getting Started Hint */}
-                  {!currentWorkflow && !isWorkflowRunning && workflows.length === 0 && (
+                  {!currentWorkflow && !isWorkflowRunning && workflowsLoaded && workflows.length === 0 && (
                     <div className="p-4 rounded-md border border-accent-blue/20 bg-accent-blue/5 flex items-start gap-3">
                       <svg
                         width="18"

frontend/src/components/PlaygroundPage.tsx

@@ -365,7 +365,7 @@ export function PlaygroundPage() {
               <Select
                 className="w-full"
                 placeholder="Select provider"
-                value={providerId}
+                value={providers.length > 0 ? providerId : undefined}
                 onChange={(val) => {
                   setProviderId(val);
                   setModelName(null);