fix: apply onwarn directly on merged config for reliable warning suppression

Instead of passing onwarn through mergeConfig (which may not properly
merge function callbacks), apply it directly on the merged config object.
This ensures the handler is always active regardless of how Storybook's
internal Vite config is structured.

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

.storybook/main.ts

@@ -19,7 +19,7 @@ const config: StorybookConfig = {
     autodocs: "tag",
   },
   async viteFinal(config) {
-    return mergeConfig(config, {
+    const merged = mergeConfig(config, {
       define: {
         'process.env': {},
         'process.platform': '"browser"',
@@ -82,21 +82,30 @@ const config: StorybookConfig = {
       },
       build: {
         target: 'esnext',
-        rollupOptions: {
-          onwarn(warning, warn) {
-            // Suppress "use client" directive warnings (from Radix UI, react-router, etc.)
-            // and sourcemap resolution errors from dependencies with incomplete sourcemaps
-            if (
-              warning.code === 'MODULE_LEVEL_DIRECTIVE' ||
-              warning.message?.includes("Can't resolve original location of error")
-            ) {
-              return;
-            }
-            warn(warning);
-          },
-        },
       },
     });
+
+    // Apply onwarn directly to avoid mergeConfig potentially dropping function callbacks
+    merged.build ??= {};
+    merged.build.rollupOptions ??= {};
+    const existingOnwarn = merged.build.rollupOptions.onwarn;
+    merged.build.rollupOptions.onwarn = (warning, warn) => {
+      // Suppress "use client" directive warnings (from Radix UI, react-router, etc.)
+      // and sourcemap resolution errors from dependencies with incomplete sourcemaps
+      if (
+        warning.code === 'MODULE_LEVEL_DIRECTIVE' ||
+        warning.message?.includes("Can't resolve original location of error")
+      ) {
+        return;
+      }
+      if (existingOnwarn) {
+        existingOnwarn(warning, warn);
+      } else {
+        warn(warning);
+      }
+    };
+
+    return merged;
   },
 };
 export default config;