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-lock.json

@@ -44,6 +44,7 @@
         "@mantine/hooks": "^8.3.1",
         "@mui/icons-material": "^7.3.2",
         "@mui/material": "^7.3.2",
+        "@posthog/react": "^1.8.2",
         "@reactour/tour": "^3.8.0",
         "@stripe/react-stripe-js": "^4.0.2",
         "@stripe/stripe-js": "^7.9.0",
@@ -2871,6 +2872,22 @@
         "cross-spawn": "^7.0.6"
       }
     },
+    "node_modules/@posthog/react": {
+      "version": "1.8.2",
+      "resolved": "https://registry.npmjs.org/@posthog/react/-/react-1.8.2.tgz",
+      "integrity": "sha512-KzUuXIcAR8fAjU7IeDq+XfEcUTNvzgEGB381WRrFUUsu7jFTcKZZ6crx/ukHRCzTnoEuy5EJDkL7b7sJecPlCg==",
+      "license": "MIT",
+      "peerDependencies": {
+        "@types/react": ">=16.8.0",
+        "posthog-js": ">=1.257.2",
+        "react": ">=16.8.0"
+      },
+      "peerDependenciesMeta": {
+        "@types/react": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/@posthog/types": {
       "version": "1.354.0",
       "resolved": "https://registry.npmjs.org/@posthog/types/-/types-1.354.0.tgz",