TICKET-019: Create onboarding flow

[alexphiev]

🚀 Polish & Launch Readiness

User Onboarding & Engagement

First-time user: welcome modal

Explain app in 3 steps:

• "Discover authentic nature places"
• "Track where you've been and want to go"
• "Rate places to help the community"

Request location permission

Optional: Quick preferences (favorite activities, difficulty level)