feat: group discovery with recommendation engine

[victorjzq]

Closes #77

Summary

• New GroupRecommendations component (src/components/GroupRecommendations.tsx) with three discovery sections:
  • Recommended for You — groups ranked by token-preference match and contribution accessibility; excludes groups the user is already a member of
  • Popular Groups — active groups sorted by activity score (member count + status + recency)
  • New Groups — Forming groups created in the last 72 hours, badges with ✨ New
• Integrated into the homepage between the "Why SoroSave?" features section and the footer
• walletAddress and userTokens props allow personalisation once contract queries are wired in

Acceptance Criteria Met

• Show recommended groups on dashboard based on token preference and contribution range
• "Popular groups" section showing most active groups
• "New groups" section showing recently created groups in Forming status
• Filter recommendations by user's wallet token balances (prop-ready for Freighter integration)

/claim #77