- Fix registration flow redirecting to backend port instead of frontend
  by using return_url (consistent with login/link flows)
- Allow HTTP redirects during OIDC discovery and token exchange so
  providers behind reverse proxies (e.g. Authelia) work correctly
- Improve error message when auth URL generation fails to include
  provider slug and troubleshooting hint

- Add contacts to Onboarding list (ID 12) alongside Product Updates
  when a new organization is created

Use window.location.origin instead of window.location.href for the
return_url, consistent with the login modal. Prevents redirecting
back to /onboarding after registration.

Add onboarding, activation, and engagement telemetry events to the
PostHog subscriber filter so they appear in analytics dashboards:
- Onboarding: OrgCreated, OnboardingModalCompleted, PlanSelected
- Activation: FirstDaemonRegistered, FirstDiscoveryCompleted,
  FirstHostDiscovered, FirstTopologyRebuild
- Engagement: SecondNetworkCreated, FirstTagCreated,
  FirstUserApiKeyCreated, FirstSnmpCredentialCreated,
  InviteSent, InviteAccepted

- Spawn non-interfaced IP sender in tokio::spawn so it runs
  concurrently with the select! consumer, preventing deadlock when
  >256 IPs fill the bounded channel
- Skip pnet is_up() check on Windows where flags are hardcoded to 0,
  rely on datalink::channel() as the real capability test
- Use most-specific subnet match in get_own_interfaces to prevent
  0.0.0.0/0 catch-all from stealing MAC mappings