Add Mixpanel tracking and activation funnel analytics

[mcybar]

We need product analytics on the ClawVoice setup and call flow so we can see where users drop off and which parts of the voice pipeline fail.

What to add

• Initialize Mixpanel behind config/env flags.
• Identify users and workspaces with stable IDs.
• Track core lifecycle events from onboarding and call handling.
• Define funnels for activation and successful call completion.

Suggested events

• app_opened
• onboarding_started
• provider_configured
• call_requested
• call_started
• call_answered
• call_completed
• call_failed
• transcript_saved
• webhook_error

Suggested funnels

• Activation: app_opened -> provider_configured -> call_requested -> call_completed
• Call reliability: call_requested -> call_started -> call_completed
• Failure diagnosis: call_requested -> call_failed

Event properties

• user_id, workspace_id
• call_id
• provider, voice_provider
• call_direction
• status
• error_code
• duration_ms
• destination_country
• model or voice identifiers where relevant

Requirements

• Keep tracking disabled unless config/env explicitly enables it.
• Do not send raw PII.
• Add tests or mocked assertions for the main event paths.
• Document the event taxonomy so future funnel changes stay consistent.

Success criteria

• Mixpanel can be enabled and disabled cleanly.
• Key events are emitted from the onboarding and call flow.
• The activation funnel can be built from the emitted events without extra backend work.