feat(admin): add admin panel for group management

[aunysillyme]

/claim #11

Summary

• Adds AdminPanel.tsx with admin-only controls: Start Group (Forming → Active), Pause/Resume, Remove Member, Transfer Admin Role, and Emergency Withdraw with an inline confirmation step.
• Mounts the panel on groups/[id]/page.tsx next to the existing actions card. The panel only renders when the connected wallet matches the group admin.
• Each action follows the existing pattern (build tx via sorosaveClient, sign with Freighter, log the signed XDR pending tx-submission TODO that already exists elsewhere in the codebase).

Notes

• This PR is AI-assisted (Claude).
• The removeMember, transferAdmin, startGroup, pauseGroup, resumeGroup, and emergencyWithdraw calls on sorosaveClient follow the naming/signature conventions used by contribute and createGroup. If the SDK method names differ, they're a one-line rename per action.
• No new dependencies. Stays consistent with existing Tailwind styles and the wallet/signing flow.

Test plan

• Connect as the group admin → admin panel renders.
• Connect as a non-admin → panel is hidden.
• Status Forming: Start Group button appears and triggers a sign request.
• Status Active: Pause button appears.
• Status Paused: Resume button appears.
• Remove Member: dropdown excludes the admin; submit triggers sign request with selected member.
• Transfer Admin: address field validates non-empty; submit triggers sign request.
• Emergency Withdraw: first click reveals confirmation; Confirm triggers sign request, Cancel dismisses.

Closes #11