feat: Add onboarding tour for new users

[huangxiao2008]

Closes #72

Problem Summary:
New users needed a guided tour to understand the key features of the SoroSave app (wallet connection, browsing groups, joining, and contributing).

Solution Approach:

• Implemented a step-by-step tour using react-joyride.
• Added specific class names (tour-wallet-connect, tour-browse-groups, tour-join-group, tour-contribute) to key interactive elements across the application to act as targets for the tour.
• Created the OnboardingTour component and wrapped the main application inside providers.tsx.
• The tour state is saved in localStorage so it only runs once per user automatically, but can be triggered manually if needed later.

/claim #72