[contracts] Implement Treasury.initialize()

[Ehonrie]

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

What to implement:

• Panic if already initialized
• Store ADMIN and FACTORY addresses
• Set BALANCE=0, TOTAL_FEES_EARNED=0
• Initialize WITHDRAWAL_LOG as empty Vec

Acceptance Criteria:

• Double-init panics with descriptive error
• get_balance() returns 0 after init
• Unit test covers happy path and double-init

[lettalkdata-cloud]

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

I will love to work on this issue

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[drips-wave]

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

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