Only validate env vars on dev server, not build

CI has no .env file. Validate on serve only and default to empty
strings during build so CI passes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ksroda-sa

samples/react/login-pkce/vite.config.ts

@@ -9,20 +9,22 @@ const exposedEnvVars = [
   "SCOPES",
 ];
 
-export default defineConfig(({ mode }) => {
+export default defineConfig(({ mode, command }) => {
   const env = loadEnv(mode, process.cwd(), "");
-  const missing = exposedEnvVars.filter((key) => !env[key]);
-  if (missing.length > 0) {
-    throw new Error(
-      `Missing required env vars: ${missing.join(", ")}. Copy .env.example to .env and fill in the values.`,
-    );
+  if (command === "serve") {
+    const missing = exposedEnvVars.filter((key) => !env[key]);
+    if (missing.length > 0) {
+      throw new Error(
+        `Missing required env vars: ${missing.join(", ")}. Copy .env.example to .env and fill in the values.`,
+      );
+    }
   }
   return {
     plugins: [react()],
     define: Object.fromEntries(
       exposedEnvVars.map((key) => [
         `import.meta.env.${key}`,
-        JSON.stringify(env[key]),
+        JSON.stringify(env[key] ?? ""),
       ]),
     ),
     server: {