feat: add PostHog telemetry provider for cloud builds

- Add posthog-js dependency
- Create PostHogTelemetryProvider following MixpanelTelemetryProvider pattern
- Register provider in initTelemetry.ts
- Read posthog_project_token and posthog_api_host from window.__CONFIG__ (runtime config from /features endpoint)
- Tree-shaken away in OSS builds (verified)

Author: robinjhuang

global.d.ts

@@ -33,6 +33,8 @@ interface Window {
     gtm_container_id?: string
     ga_measurement_id?: string
     mixpanel_token?: string
+    posthog_project_token?: string
+    posthog_api_host?: string
     require_whitelist?: boolean
     subscription_required?: boolean
     max_upload_size?: number

package.json

@@ -100,6 +100,7 @@
     "loglevel": "^1.9.2",
     "marked": "^15.0.11",
     "pinia": "catalog:",
+    "posthog-js": "catalog:",
     "primeicons": "catalog:",
     "primevue": "catalog:",
     "reka-ui": "catalog:",