[contracts] Implement Treasury.withdraw_fees()

[Ehonrie]

Implement withdraw_fees() in contracts/treasury/src/lib.rs.

What to implement:

• admin.require_auth() as first call
• Verify amount <= BALANCE, panic otherwise
• Deduct amount from BALANCE, transfer XLM to recipient
• Append (recipient, amount, timestamp) to WITHDRAWAL_LOG
• Emit FeesWithdrawn event

Acceptance Criteria:

• Non-admin panics
• Panics if amount exceeds balance
• Balance decreases correctly
• Withdrawal logged in WITHDRAWAL_LOG
• Unit tests cover happy path and all errors

[dinmacmd]

@dinmacmd has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @dinmacmd to this issue.

[drips-wave]

Congratulations, @dinmacmd! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @dinmacmd: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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