Add feature flag detection for profile redesign (#37472)

app/javascript/mastodon/utils/environment.ts

@@ -12,8 +12,21 @@ export function isProduction() {
   else return import.meta.env.PROD;
 }
 
-export type Features = 'fasp' | 'http_message_signatures';
+export type ServerFeatures = 'fasp';
 
-export function isFeatureEnabled(feature: Features) {
+export function isServerFeatureEnabled(feature: ServerFeatures) {
   return initialState?.features.includes(feature) ?? false;
 }
+
+type ClientFeatures = 'profile_redesign';
+
+export function isClientFeatureEnabled(feature: ClientFeatures) {
+  try {
+    const features =
+      window.localStorage.getItem('experiments')?.split(',') ?? [];
+    return features.includes(feature);
+  } catch (err) {
+    console.warn('Could not access localStorage to get client features', err);
+    return false;
+  }
+}