[Frontend] [Feature] Integrate @stellar/freighter-api to sign mint_certificate

[Tybravo]

🎯 Goal

Prompt the user's Freighter wallet to sign the final minting transaction.

📋 Requirements

• Implement the feature as described in the goal.
• Ensure integration matches the architecture defined in SUMMARY_TYPESCRIPT.md.
• Handle relevant loading and error states gracefully.

🔧 Technical Scope

• @stellar/freighter-api

📂 Affected Path

frontend/hooks/useSignTransaction.ts

✅ Acceptance Criteria

• Freighter popup appears and accepts the transaction.

🔗 PR Requirements

• PR description must include: Closes #[issue_id]