UI onboarding screens

[cryptoquick]

I implemented the three screens from styles and graphics generated from the Figma. To test the screens if you've already set up your wallet, you can prefix cargo run with TESTING=1 like this:

TESTING=1 cargo run

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Add onboarding screens with modern UI design

This PR adds a complete, user-friendly onboarding flow to BitVault Wallet with three screens that introduce key concepts to new users:

1. Multisig Security - Explains the 2-of-3 multisig vault concept with recommendations for securing keys across different geolocations
2. Time-delay Protection - Highlights time-delay features that prevent unauthorized withdrawals and emphasizes xPUB importance
3. Secret Notifications - Introduces end-to-end encrypted notifications for wallet events and security updates

Technical details:

• Added 3 new SVG assets (onboarding1-3.svg) for illustrations
• Implemented arrow key navigation between screens
• Added clickable dot indicators for screen navigation
• Designed with a modern, clean UI with consistent styling
• Added "onboarding_completed" flag to track user progress
• Implemented responsive layout that works across screen sizes

The onboarding flow enhances user experience and security awareness before users create or import a wallet.