feat: add client-side PostHog telemetry tracking (#37442)

Adds client-side PostHog tracking to run in parallel with server-side telemetry across studio, docs, and www. This enables session replays and resolves a race condition where page views arrive before group assignments resulting in attribution errors.

Changes:
- Created PostHog client wrapper with consent-aware initialization in common package
- Integrated PostHog client calls into existing telemetry functions to send events to both PostHog (client) and backend (server)
- Updated CSP to allow connections to PostHog endpoints
- Added environment variable support for all apps
- PostHog client accepts consent as a parameter and respects user preferences
- Events can be distinguished in PostHog by $lib property (posthog-js vs posthog-node)
- PostHog URL configured based on environment (staging/local uses ph.supabase.green)
- Maintains full backward compatibility with existing telemetry system

Resolves GROWTH-438
Resolves GROWTH-271

packages/common/package.json

@@ -13,12 +13,13 @@
   "dependencies": {
     "@types/dat.gui": "^0.7.12",
     "@usercentrics/cmp-browser-sdk": "^4.42.0",
-    "flags": "^4.0.0",
     "api-types": "workspace:*",
     "config": "workspace:*",
     "dat.gui": "^0.7.9",
+    "flags": "^4.0.0",
     "lodash": "^4.17.21",
     "next-themes": "^0.3.0",
+    "posthog-js": "^1.257.2",
     "react-use": "^17.4.0",
     "valtio": "catalog:"
   },