pdfReceipt.ts: Support custom logo injection into generated receipts

[Kingsman-99]

Description

src/pdfReceipt.ts generates receipts with a static header. Integrators want to embed their own logo in the receipt header. The generator should accept an optional base64-encoded image and render it in the header section.

Acceptance Criteria

• ReceiptOptions accepts an optional logoBase64: string field
• When provided, the logo is rendered in the top-left of the receipt header
• When omitted, the existing static header is rendered unchanged
• Unit tests pass

Context

• Target file: src/pdfReceipt.ts
• Logo must be validated as a non-empty string before use; throw InvalidLogoError if validation fails

[techisigu]

@techisigu has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I’m a software developer with experience in JavaScript, TypeScript, React, Node.js, Rust, and blockchain development, and I’m particularly interested in contributing to open-source projects where I can solve real problems, learn from experienced contributors, and give back to the developer community.

I have reviewed the issue and believe I can contribute effectively by [briefly describe what you intend to implement/fix]. My approach would be to first understand the existing implementation and project conventions, reproduce the issue where necessary, implement the required changes, and add or update tests to ensure the solution works correctly without introducing regressions.

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[grantfox-oss]

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

@techisigu's PR #712 was approved and merged by @Kingsman-99.

🏆 @techisigu: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (144 total points). Track your full progress on GrantFox.

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

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This issue has been completed by @techisigu as part of the Stellar Wave Program's 8th Wave 🥳

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