Add welcome screen on first app start

[tobiaswu]

Description

Display a welcome screen on first app launch after installation to introduce users to Echos' core features and functionality. This improves onboarding and sets expectations for new users.

Acceptance criteria

• Screen displays only on first app launch (not on subsequent opens)
• Includes intro to app's key features and value proposition
• Provides clear call-to-action to proceed to main app
• Design matches Figma specifications
• Works on both iOS and Android

References

Figma design