Add a "click to copy prompt" option to the /onboard route's Step 2, giving users a simpler alternative to manually copying JSON config snippets.

Context

• /onboard (OnboardingFlow.tsx) has a 3-step flow; Step 2 shows JSON config snippets for Claude Code, Cursor, and Qwen with copy buttons
• /r/welcome (ExploreWithAICard.tsx) has a simpler "click to copy prompt" pattern that copies a natural language prompt to clipboard
• Same clipboard copy pattern can be reused

Decisions Made

1. Placement: Above the client tabs as a prominent card (e.g. "Or, copy this prompt and paste it into your AI assistant")
2. Prompt content: Self-contained — includes the user's API key and direct config instructions telling the AI exactly what JSON to write and where. No external URL dependency.
3. Scope: Single generic prompt that works for any AI client (not client-specific)

Scope

In scope:

• New card/section above client selector tabs in Step 2 of OnboardingFlow
• Generate a natural language prompt containing the user's API key and MCP config instructions
• Copy-to-clipboard button with copied feedback state

Out of scope:

• Refactoring existing copy utilities into shared components
• Changes to /r/welcome
• Client-specific prompt variants