Switch UI build system to pnpm

[MindTooth]

I've changed the setup to pnpm as more and more projects are doing the same. I've noted quite an increase in speed.

Also includes some other tweaks:

• fixed legacy Tailwind option
• inlined all resources
• bump prettier
• tweaked the documentation a bit (this needs a thorough rewrite)
• moved types to regular deps (same package in same group)

Instruction for publishing

cd src/ui
pnpm install
pnpm run build:prod

Fixed #66.

[MindTooth]

I think it's done now.

I'll work on the release pipeline next. But, first I'll see if I can tweak the running of Flask with watch or something. So it's easier to developer at the same time both Python and CSS/JS code in development mode.

[tomrtk]

Ok, will you squeeze the commits? as some of them look to not be passing ci if checked out?