Add invoice customization with per-tenant branding

[Smartdevs17]

Context\nInvoices are plain, unbranded documents. Enterprise merchants need customized invoices that match their brand.\n\n## Current Limitation/Problem\nInvoice generation uses a single template with no merchant customization.\n\n\n## Expected Outcome\nInvoice customization: merchant logo, colors, custom fields (PO number, VAT ID), legal text, and per-locale formatting.\n\n## Acceptance Criteria\n- [ ] Invoice branding: upload logo, pick primary/accent color\n- [ ] Custom fields: PO number, cost center, department\n- [ ] Legal text: footer notes, payment terms, late fee policy\n- [ ] Locale-specific: date format, currency symbol position, number format\n- [ ] PDF generation with WKHTMLTOPDF or Puppeteer\n- [ ] Invoice preview before sending\n- [ ] Edge case: long legal text overflows to second page\n- [ ] Edge case: custom field exceeds character limit\n\n## Technical Scope\n- - BrandingService, PDFGenerator\n- - branding configuration API\n- - invoice branding page\n- - base template with variable injection\n- - PDF rendering engine (Puppeteer)

[amehsamuel200225-sketch]

@amehsamuel200225-sketch has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi…. I’m a software engineer, let me fix this issue please!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @amehsamuel200225-sketch to this issue.

[drips-wave]

Congratulations, @barry01-hash! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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[drips-wave]

Congratulations, @thefifthdev! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @thefifthdev: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

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