docs: rewrite the README for any package manager (BITS-221)

[PhilReinking]

Rewrites the README from what is on main: install for npm, pnpm, yarn and bun, how an app sets up Tailwind 3, icons and a first component, and the develop and release steps.

Fixes BITS-221

• Apps install Font Awesome 6 themselves: pnpm hides a package's own dependencies, and version 6 keeps one copy shared with the components.
• The Tailwind config adds darkMode: "class", the grey color and @tailwindcss/forms, because the components use all three.

Tested: followed the README in fresh Vite + Vue 3 + Tailwind 3 apps with npm, pnpm, yarn and bun, using the packed tarball. A styled D9Button and an app-registered icon render.
Not tested: a real release.sh run; its version steps ran through npm version --no-git-tag-version.

🤖 Generated with Claude Code

[PhilReinking]

OK at 3589c68 for the content, CI green. One should, inline. I'll re-check the lint line after #26.

Checked against main: every command exists except npm run lint, the release examples give the versions shown, and release.sh and the publish workflow do what the text says. A Vite app built from this README alone on main's package shows the button and the app-registered icon.

[PhilReinking]

The components carry dark: classes, and Tailwind 3 defaults to darkMode: "media". So on a computer in dark mode, D9Input turns dark slate on the app's white page. The repo and Input use "class"; with it the components stay light until the app sets dark:

Suggested change

	export default {
	export default {
	darkMode: "class",

[PhilReinking]

Added in be13e04, with one line on why. Checked: in OS dark mode, D9Input now stays white instead of slate-800.

[PhilReinking]

OK at 09b99a8, CI green (lint, test, build).

The darkMode: "class" line is the one I tested: the components stay light on a computer in dark mode. Against main 2f64d29, every command in Develop runs here: lint (eslint . --max-warnings 0), test 10/10, check, build, and Storybook starts.