feat: implement CreateGroupModal and group detail page (#4, #9)

[Yzgaming005]

Summary

Implements two core features for the CoopFinance groups system:

Changes

• CreateGroupModal ([Frontend] Implement CreateGroupModal with full Soroban contract deployment flow #4): Full group creation form with:
  • Group name (3-50 chars, required)
  • Description (optional, max 200 chars)
  • Minimum contribution (USDC, required)
  • Cycle duration selector (7/14/30 days)
  • Validation, loading states, success/error feedback
• Group Detail Page ([Frontend] Build individual group detail page at /groups/[id] #9): route with:
  • Stats grid (balance, members, min contribution, cycle)
  • Contract details (admin, status, created date)
  • Members list with admin badge
  • Responsive layout with mock data fallback

Testing

• Form validation works (empty fields, short name)
• Create flow shows loading → success states
• Group detail page renders with mock data
• Copy address button works

Closes #4, #9

[Yzgaming005]

PR conflicting (DIRTY). Perlu rebase ke main. Juga missing lock file (package-lock.json/yarn.lock) — CI failed untuk itu. Fix: git rebase main + commit lock file, atau tambahkan .gitignore + generate lock.