[Enhancement] Onboarding wizard for new users

[8dazo]

Description

Inspired by OpenClaw's onboarding. Guide new users through initial setup.

Implementation

• Add onboarding flow screens
• Step 1: Enable Accessibility Service (direct link)
• Step 2: Set LLM API Key and Model
• Step 3: Run 'hello world' tool test

Acceptance Criteria

• Users guided to enable service
• API keys verified before finishing
• Progress indicators for each step