Add downloadable tax receipts

[kitWarse]

Summary

• Added downloadable PDF tax receipts for contributor contribution history.
• Added individual receipt downloads and a batch PDF download for all contribution receipts.
• Added a protected frontend tax receipts page linked from the contributor dashboard.

Issue

• Closes FEATURE: Tax Receipt Generation #589

Changes

• Added authenticated receipt listing and PDF download endpoints under /api/contributions/tax-receipts.
• Added dependency-free PDF generation for contribution receipt records.
• Added frontend API helpers for receipt listing and downloads.
• Added /tax-receipts page and linked it from the contributor dashboard.

Validation

• Ran node --check backend/src/routes/contributions.js.
• Ran node --check backend/src/services/taxReceiptPdf.js.
• Ran git diff --check.
• Did not run full lint/test/build because dependencies are not installed in this fresh clone and the request was to PR after required changes without CI cleanup.

Notes

• Receipts include contribution amount, date, campaign details, contributor details, and transaction hash.
• Batch download generates one PDF with one receipt page per contribution.

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