Posthog-fixes (#5901)

PostHog is now initialized with persistence: 'memory' so no cookies are
written on first load. Consent is handled in a PostHogConsentSync
component that switches to localStorage+cookie persistence only when the
user accepts, using the official @posthog/react package (cherry-picked
from 14aaf64)

---------

Co-authored-by: James Brunton <[email protected]>

frontend/src/global.d.ts

@@ -26,16 +26,4 @@ declare module 'axios' {
   }
 }
 
-declare module 'posthog-js/react' {
-  import { ReactNode } from 'react';
-  import posthogJs, { PostHogConfig } from 'posthog-js';
-
-  export const PostHogProvider: React.FC<{
-    client?: typeof posthogJs;
-    options?: Partial<PostHogConfig>;
-    apiKey?: string;
-    children?: ReactNode;
-  }>;
-}
-
 export { };