feat: Openclaw migration dialog (first-launch onboarding)

[qiongyu1999]

Summary

• Adds StepOpenclawMigration.tsx — a first-launch popup that detects existing Openclaw config (~/.openclaw/memory/ and ~/.openclaw/skills/) and offers one-click migration to Nexu
• Wires the new step into JourneyPage.tsx as step 2 (between Landing and Onboarding)
• Four-phase flow: Scan → Select → Migrating → Done, with animated progress and per-item toggle

What the popup does

1. Scan phase — animated progress bar while "detecting" local Openclaw files
2. Select phase — checklist of discovered memory files and skill packages; user can deselect any item
3. Migrating phase — item-by-item migration animation
4. Done phase — confirmation with destination paths (~/.nexu/memory/, ~/.nexu/skills/) and a "Start using Nexu" CTA

Original Openclaw files are explicitly noted as untouched.

Test plan

• Open /journey and navigate to step 2 ("迁移 Openclaw")
• Verify scan animation auto-advances to select screen
• Toggle items on/off; confirm button count updates
• Click "一键迁移" — verify item-by-item animation then done screen
• Click "跳过，手动配置" — dialog closes
• Click ✕ — dialog closes, "重新演示" button appears
• Build passes: pnpm --filter demo build ✓

🤖 Generated with Claude Code

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