feat(export): add CSV export endpoint and utility for grant data (#161)

[s6pa1rta3n-lab]

Summary

Closes #161.

Implements secure, RFC 4180-compliant CSV export for grant/task records across the API and frontend components.

Acceptance Criteria Fulfillment

• CSV Format Output: Generates valid RFC 4180 CSV with escaped quotes, commas, and linebreaks.
• Defined Public Fields: Clearly exports grant metadata (id, title, description, rewardStroops, rewardXlm, deadlineUnix, deadlineIso, maxSubmissions, submissionCount, status, createdAt, difficulty, technologies, organization, posterAddress).
• Security & Sanitization: Excludes sensitive/restricted internals and protects against CSV formula injection (DDE attacks) by escaping leading =, +, -, @, \t, \r characters.
• API Route: Added GET /api/tasks/export/csv with rate-limiting headers and filter query param support.
• UI Component: Added accessible CsvExportButton component with loading and disabled states.
• Verification: Comprehensive unit and integration test coverage across all test suites (321 frontend tests + 155 contract tests passing).

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC