Add macOS-first first-run onboarding and provider setup #65

Goal

Help a new user reach a working first diagram without having to discover provider setup or core controls on their own.

Scope

• Show a concise first-run welcome flow on macOS
• Explain the basic Lucid workflow, then guide provider setup
• Offer clear paths for LM Studio, Ollama, and OpenAI-compatible/cloud providers
• Show where API keys are saved and check whether a local provider is reachable
• Use restrained setup progress and transition animation, with reduced-motion support
• Finish with a small guided tour of the first useful actions
• Let users skip the flow and reopen it later from Settings

Boundaries

• Do not install or launch third-party model software silently
• Do not require an AI provider to use non-AI parts of Lucid
• Keep provider secrets in the existing secure credential path
• Build the flow so later Windows/Linux adaptation is straightforward

Acceptance criteria

• First launch has an accessible, keyboard-usable onboarding flow
• Provider setup reuses existing settings and connection checks
• Completion/skip state persists
• The flow can be reopened
• Focused tests cover first-run visibility, persistence, and provider handoff
• Existing app behavior remains unchanged for returning users