Add/extend batch verifier tests for header parsing, filtered manifest export, and export UX

[multiplex55]

Motivation

• Improve unit and integration coverage for the batch verifier feature by validating header parsing, filtered manifest computation, and export UX to prevent regressions in buy-plan adjustments and export formatting.

Description

• Added header parsing tests to parsing.test.ts to verify buy/sell station parsing, numeric parsing of comma-formatted jumps and cargo values, and safe handling when header lines are missing.
• Added a new filteredManifest.test.ts that verifies rows with states do_not_buy, quantity_mismatch, and missing_from_export are excluded, that original manifest order is preserved for safe rows, that rejected rows are excluded from the multibuy block, and that totals (items, capital, gross sell, volume, ISK/jump) and adjusted-profit calculations are recomputed using export buy values (Option B) with correct formatting.
• Extended BatchBuyVerifier.test.tsx to assert adjusted buy-plan metrics render when exclusions exist, that the Export action copies the filtered manifest (only safe rows, preserved order), that export is disabled when no safe rows exist, and that duplicate-name warnings disable export.
• Files changed: frontend/src/features/batchVerifier/__tests__/parsing.test.ts, frontend/src/features/batchVerifier/__tests__/filteredManifest.test.ts (new), and frontend/src/features/batchVerifier/__tests__/BatchBuyVerifier.test.tsx (extended).

Testing

• Ran the targeted frontend test suite for the modified files with npm -C frontend run test -- src/features/batchVerifier/__tests__/parsing.test.ts src/features/batchVerifier/__tests__/filteredManifest.test.ts src/features/batchVerifier/__tests__/BatchBuyVerifier.test.tsx.
• All test files passed: 3 test files, 36 tests total, all passing.

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Codex Task