chore(analytics): register surface: 'app' super-property

app/src/core/analytics/PostHogProvider.tsx

@@ -11,7 +11,7 @@
  */
 
 import React from 'react';
-import { PostHogProvider as PHProvider } from 'posthog-react-native';
+import { PostHogProvider as PHProvider, usePostHog } from 'posthog-react-native';
 import { useConsentStore } from '@/core/stores/consentStore';
 import { env } from '@/core/config/env';
 
@@ -23,6 +23,26 @@ interface PostHogProviderProps {
   children: React.ReactNode;
 }
 
+/**
+ * Tags every PostHog event with `surface: 'app'` so the app's data
+ * stays distinguishable from being-website's data in the shared PostHog
+ * project (free-tier constraint: 1 project per account). The website
+ * mirrors this with `ph.register({ surface: 'web' })` in its own
+ * PosthogProvider — see mp2ez/being-website#42.
+ *
+ * Renders inside <PHProvider> so `usePostHog()` returns the initialized
+ * instance. Runs once when the instance becomes available.
+ */
+function RegisterSurfaceProperty(): null {
+  const posthog = usePostHog();
+  React.useEffect(() => {
+    if (posthog) {
+      posthog.register({ surface: 'app' });
+    }
+  }, [posthog]);
+  return null;
+}
+
 /**
  * PostHog Provider Component
  *
@@ -69,6 +89,7 @@ export function PostHogProvider({ children }: PostHogProviderProps): React.React
         captureAppLifecycleEvents: false, // We handle this ourselves
       }}
     >
+      <RegisterSurfaceProperty />
       {children}
     </PHProvider>
   );