[28] Replace inline `require_admin` boilerplate with a shared helper

[merlik787-droi]

Problem Statement

Every contract repeats the same five-line pattern in admin-only functions:

admin.require_auth();
let stored_admin: Address = env.storage().instance().get(&DataKey::Admin).expect("...");
assert!(admin == stored_admin, "Unauthorized");
// ... business logic

This is verbose, prone to copy/paste error (the auth check is sometimes assert!, sometimes panic!("Unauthorized"), with random string variations), and lacks a structured ContractError::Unauthorized enum value.

Why It Matters

• Inconsistent error strings across contracts (some say "Unauthorized", others "Unauthorized: Caller is not the protocol admin", others "Contract not initialized", others "Not initialized") make off-chain error handling fragile.
• Boilerplate crowding obscures business logic.

Expected Outcome

• A shared helper contracts_common::auth::require_admin(env, expected: &Address).
• A ContractError enum with explicit variants.
• Migration of all six contracts.

Acceptance Criteria

• grep -r "Unauthorized" contracts/*/src/lib.rs returns only references inside the (now centralized) error enum.
• All tests still pass.
• New tests for the helper edge cases.

Implementation Notes

• Use contracterror! macro from soroban-sdk for proper error encoding.
• Tie to Issue [7] create_contracts_common shared crate — eliminate duplicated types and patterns #7 (common crate).

Files / Modules Affected

All six contracts; new contracts/common/src/auth.rs and errors.rs.

Dependencies

Issue #7.

Difficulty

Medium.

Estimated Effort

1–2 days.

Suggested Labels

quality, refactor, P2, tech-debt

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[Nabeelahh]

Hi maintainer I have thoroughly read the description to this issue and I can resolve this according to the description required ensuring it meet all the acceptance criteria kindly assign.

[grantfox-oss]

🦊 GrantFox — @Nabeelahh has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #28)
2. Your PR will be reviewed by the Kqirox maintainers

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