Build invoice batch duplicate-detection before creation

[Kingsman-99]

Description

src/app/invoice/batch/page.tsx creates multiple invoices at once with no check for accidental duplicates (same recipient + amount entered twice). This issue adds pre-submission duplicate detection.

Technical Context

Involves src/app/invoice/batch/page.tsx. Before submission, scan the batch list for entries with identical recipient+amount+token combinations and surface a warning requiring explicit confirmation to proceed.

Acceptance Criteria

• Detects exact-duplicate rows (same recipient, amount, and token) within the same batch submission
• Shows a warning listing the duplicate row numbers, requiring an explicit "Submit anyway" confirmation
• Does not block submission outright — duplicates may be intentional (e.g. recurring fixed amounts)
• Does not flag legitimately different rows (different amount or token) as duplicates
• Unit tests: exact-duplicate detection, near-duplicate (different amount) non-detection

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[Austinaminu2]

@Austinaminu2 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i've scoped this issue and i'm sure i can finish it quickly

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[firstJOASH]

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

i can solve this issue
kindly assign me

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[grantfox-oss]

🦊 GrantFox — @Austinaminu2 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #254)
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[drips-wave]

Congratulations, @Austinaminu2! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Austinaminu2: 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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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@Austinaminu2's PR #275 was approved and merged by @Kingsman-99.

🏆 @Austinaminu2: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (331 total points). Track your full progress on GrantFox.

👏 Great work, @Austinaminu2! Keep contributing to Stellar-split.

[drips-wave]

This issue has been completed by @Austinaminu2 as part of the Stellar Wave Program's 6th Wave 🥳

😎 @Austinaminu2: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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