feat: build individual group detail page at /groups/[id] (Closes #9)

[waterWang]

Summary

Creates the missing /groups/[id] route that was previously returning a 404. The Groups list page has "View Group" links pointing to /groups/[id] — this PR implements the detail page those links navigate to.

Changes

New file: src/app/groups/[id]/page.tsx

Header:

• Group name, description, status badge (Active/Inactive)
• Admin address (shortened with copy-to-clipboard button)
• Creation date and member count
• "Add Member" button visible only to the group admin

Stats Row (4 cards):

• Treasury balance (USDC on Stellar, with live Soroban RPC TODO)
• Total members (with active count)
• Active loans (with total count)
• Total contributions (lifetime)

Four tabs using @radix-ui/react-tabs:

Tab	Content
Members	Table with display name, address, contributed amount, loan balance, join date, active status
Contributions	Table with member, amount, period, date, tx hash (linked to Stellar Expert)
Loans	Grid of existing LoanCard components filtered to this group
Governance	Grid of existing ProposalCard components filtered to this group

States handled:

• Loading state (skeleton animation)
• 404 state (with "Browse Groups" link)
• Empty state per tab
• Pre-existing TypeScript errors are unchanged (layout.tsx Toaster import error)

Data fetching:

• React Query for all data with configurable stale/refetch times
• Mock data fallback with // TODO: connect to API comments
• API endpoints: GET /api/groups/:id, /members, /contributions, /loans, /proposals

Responsive:

• Mobile-first layout with Tailwind breakpoints
• Horizontal scroll on tables for small viewports
• Stacked layout on mobile, grid on desktop

Testing

• npx tsc --noEmit passes (pre-existing error in layout.tsx only)
• All existing tests pass

Closes #9