feat(webapp): upgrade posthog-js and capture onboarding, organization, and project analytics

Upgrade posthog-js in the webapp from 1.93.3 to 1.407.3. The
`posthog.init` options pin the client defaults that changed between
these versions — person profiles, persistence, heatmaps, dead-click
capture, exception capture, web-vitals capture, and conversations — so
what the webapp captures is unchanged by the upgrade itself.

Add server-side product-analytics for the managed cloud service: the
onboarding form answers (referral source and role), an "organization
created" event (with company domain and size when provided), and a
"project created" event. These new events are collected on the managed
cloud only — self-hosted installations continue to send the same
telemetry as before.

Mono-RevId: ae703b3df9c3288cc002559f6e8f75c4e2451c33