feat(analytics): auto page view tracking + key events #3770

Context

Phase 4 PostHog — depends on PostHog fix issue.

What

• Router afterEach hook: posthog.capture('$pageview', { path, title })
• Capture key business events: signup_completed, org_created, plan_upgraded, invitation_sent
• Events captured in relevant stores/composables

Acceptance criteria

• Router afterEach captures pageview with route path and name
• Key events captured at appropriate points
• No-op if posthog not initialized
• Unit tests