feat: add merchandise page with T-shirt details

[shenxianpeng]

• Add high-quality images
• Take high-definition try-on photos
• Process orders and collect payments (international)

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[shenxianpeng]

I don’t really expect many people to buy our T-shirts — it’s more of a small way to help spread the project and make it a bit more visible in the community.

@2bndy5 By the way, when you have some time, could you also share your phone number with me via email? It’s required when I fill in your address to ship to you.

[shenxianpeng]

@copilot resolve the merge conflicts in this pull request

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[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts have been resolved. The only conflict was in mkdocs.yml under markdown_extensions — I kept pymdownx.details from this branch and merged it with the updated extension list from main (which added attr_list, md_in_html, pymdownx.highlight, pymdownx.inlinehilite, and pymdownx.snippets). Merge commit: 3550917.

[shenxianpeng]

I don't expect who will donate and will get a t-shirt, but it's better than nothing.