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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