feat: add onboarding screen (#651)

[Udhay-Adithya]

PR Description

This PR adds an Onboarding Screen to the mobile version of the API Dash. If the user is launching the app for the first time, they will be directed to the onboarding flow before accessing the main dashboard.

Changes

1. Implemented OnboardingScreen() with three slides containing Lottie animations, titles, and descriptions and a button to skip all slides.
2. Added carousel_slider for smooth and swipe-based scrolling.
3. Updated DashApp to check onboarding status using SharedPreferences.
4. Ensured onboarding is only triggered on mobile (kIsMobile).

Preview

Lottie Files Used:
api_server.json
files.json
generate.json

Related Issues

• Closes
  Feature Request: Onboarding Screen for Mobile UI #651

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: I will add test cases once the pr is reviewed

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux