feat: add invoice template system with custom branding

[yuliuyi717-ux]

Fixes #240

Summary

• Implement the requested issue behavior in the target module
• Keep changes focused and minimal
• Update tests to validate the new behavior

Validation

• Executed targeted local validation during development

[vercel]

@yuliuyi717-ux is attempting to deploy a commit to the david's projects Team on Vercel.

A member of the Team first needs to authorize it.

[yuliuyi717-ux]

Hi team, gentle bump on this one when you have a moment. I added the invoice template system with custom branding in #278 and would really appreciate a review whenever it fits your schedule. Happy to make any changes needed. Thanks!

[yuliuyi717-ux]

Hi! The Vercel bot says the deployment needs authorization.
Could someone from the team approve the deploy?

Thanks!