feat: add health check API and handle app configuration warnings

- Removed the old index.html file from the frontend build.
- Integrated a health check query in App component to verify app configuration.
- Display a warning alert if the app is not configured with AZURE_SUBSCRIPTION_ID.
- Updated API module to include health check endpoint.

app-new/frontend/src/App.tsx

@@ -36,6 +36,7 @@ const App = () => {
   const [skuExport, setSkuExport] = useState("");
   const [renderError, setRenderError] = useState("");
 
+  const healthQuery = useQuery({ queryKey: ["health"], queryFn: api.health });
   const locationsQuery = useQuery({ queryKey: ["locations"], queryFn: api.locations });
   const publishersQuery = useQuery({
     queryKey: ["publishers", selection.location],
@@ -127,6 +128,8 @@ const App = () => {
   };
 
   const loading = locationsQuery.isLoading || templatesQuery.isLoading;
+  const healthError = healthQuery.error instanceof Error ? healthQuery.error.message : "";
+  const appNotConfigured = healthError.includes("Missing AZURE_SUBSCRIPTION_ID");
   const locationsError = locationsQuery.error instanceof Error ? locationsQuery.error.message : "Failed to load locations";
 
   return (
@@ -140,10 +143,16 @@ const App = () => {
             Select a marketplace image and generate reusable snippets.
           </Typography>
 
+          {appNotConfigured ? (
+            <Alert severity="warning">
+              App is not configured. Set AZURE_SUBSCRIPTION_ID (and Azure credentials) in the container start environment, then restart the app.
+            </Alert>
+          ) : null}
+
           {loading ? (
             <CircularProgress />
           ) : locationsQuery.isError ? (
-            <Alert severity="error">{locationsError}</Alert>
+            <Alert severity={appNotConfigured ? "warning" : "error"}>{locationsError}</Alert>
           ) : (
             <Card>
               <CardContent>

app-new/frontend/src/api.ts

@@ -18,6 +18,7 @@ const json = async <T>(path: string): Promise<T> => {
 };
 
 export const api = {
+  health: () => json<{ status: string; message?: string }>("/api/health"),
   locations: () => json<LocationOption[]>("/api/locations"),
   publishers: (location: string) => json<string[]>(`/api/publishers?location=${encodeURIComponent(location)}`),
   offers: (location: string, publisher: string) =>