[frontend] Refactor CSS, split into files, update templates

[f0x52]

Description

Bit of a hellish PR, with the frontend refactor I started with over the summer. Checked pretty much everything on the testrig for correctness, only thing I didn't was themes.
Includes new set of eslint rules which mesh with typescript, but have every rule set to warn instead of error with a plugin, to be followed up with a separate PR that includes lots of linting fixes.

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Hell yeah nice one!!! Will get this reviewed!

[tsmethurst]

I finally had a look through and it looks good to me! With the different indentation now used in the HTML you'll have to change the four spaces here to a tab instead:

gotosocial/internal/router/template.go

Line 296 in 3015436

indentStr = " "