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/package.json

@@ -61,6 +61,7 @@
     "license-report": "^6.8.0",
     "pdfjs-dist": "^5.4.149",
     "peerjs": "^1.5.5",
+    "@posthog/react": "^1.8.2",
     "posthog-js": "^1.268.0",
     "qrcode.react": "^4.2.0",
     "react": "^19.1.1",