Draft: feat: signup button integration

This PR introduces some simple CSS and HTML changes to support multiple crowdfunding "providers". Namely, signup.cash, an open-source SLP-enabled wallet.

"Providers" is what I'm calling the necessary code to connect with some external wallet since I didn't see any existing terminology.

Personally, I'm curious when thinking about how to support future changes regarding providers because I'd like to meet my need for competence when delivering changes, God willing. So I'd appreciate if I could get some feedback on how to support future changes regarding "providers".

Some strategies to support future changes, iA, may include build time environment flags or runtime options in the create page. build time probably easiest for the maintainers of the project, whereas the latter might be easier for end users and not further complicate deployments.

Relevant PR's in the signup.cash repository for review:

https://github.com/signupcash/signup-core/pull/65 https://github.com/signupcash/signup-core/pull/66 https://github.com/signupcash/signup-core/pull/67 https://github.com/signupcash/signup-core/pull/68 https://github.com/signupcash/signup-core/pull/69