Add NFT total supply cap

[Samuel1-ona]

Support a maximum total supply of NFTs that can ever be minted.

Acceptance Criteria:

• Set max_supply during initialization
• Check before each mint
• get_remaining_supply query
• 0 means unlimited

[ndyugwu]

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

i would like to work on this kindly assign me

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

[drips-wave]

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

🧑‍💻 @watifee: 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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