Emergency pause and withdrawal mechanism

[AbolareRoheemah]

Closes #24

Summary
Adds an emergency pause mechanism so the admin can halt the contract if a bug is found or the group needs to be frozen, plus a trustless escape hatch so members aren't stuck if the admin goes unresponsive.

Changes

• DataKey::IsPaused (instance storage, defaults to false)
• pause() / unpause() — admin-only
• Guard added to contribute(), payout(), add_member(), reset_cycle() — all panic with "Contract is paused for emergency" while paused
• emergency_withdraw() — callable only while paused; lets a member reclaim exactly what they contributed in the current cycle (tracked via new MemberState.current_cycle_contribution field), and decrements CycleMemberCount so pool math stays valid if the contract is later unpaused
• Payout order (NextPayoutIndex) is intentionally untouched by emergency withdraw — it only unwinds contribution/pool state, it doesn't let anyone jump the rotation queue

Tests added

• Admin pause blocks contribute
• Unpause restores normal contribute flow
• Non-admin pause call fails auth
• Emergency withdraw refunds exactly the member's contribution, adjusts CycleMemberCount, and rotation/payout math still checks out after unpause
• Double emergency-withdraw in the same cycle panics

[coderabbitai]

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[Queenode]

Thank you for your contribution @AbolareRoheemah