managed-base-css-defaults

From the n1x owner, after we found that the thin scrollbar rule just added to the web plugin's `globals.css` only reaches apps generated from now on:

> do u think we should have some way to have css like that that propogaes … takign step back and thinking about right setup with n1x here

> ok lets try - i want to do cleanly

The web plugin keeps the theme tokens (`--accent*`, `--danger*`, `--success`, `--warning`, `--app-*`, in light and dark) and its base rules in `src/app/globals.css`, which is a seed file: `n1x apply` writes it once and never again. So nothing added there reaches existing apps, and apps generated before a token existed don't have it, even though managed `_lib` code reads it.

- Ship the seed's defaults, its token values for both themes and its base rules, as CSS that n1x manages, so `n1x apply` brings them to existing apps without touching their `globals.css`. Apart from the fonts below, a new app should look as it does today.
- An app's own `globals.css` must keep winning: a token or rule an app defines overrides the managed default, in both themes, and the managed CSS never overrides a Tailwind utility class (the theme switch's brief transition suppression aside).
- From the audit of the existing apps: none of them has the seed's catch-all color transition (`* { transition: … }`). Shipping it would add a transition to every element on their next apply, and it overrides Tailwind's transition utilities, so drop it, from new apps too.

> im ok with adjusting the font in the apps … im thinking geist

The root layout loads Geist and Geist Mono, but body text never renders in Geist: it falls back to the system font, even in apps whose `globals.css` maps Tailwind's fonts to the layout's Geist variables. Make body text render in Geist, in new apps and, after `n1x apply`, in existing ones, and monospace text in Geist Mono in new apps and in apps that map Tailwind's mono font to the layout's Geist Mono variable.

## Environment

This is the n1x repository itself: it has no app or database of its own to
run. `node src/cli.mjs apply --project <dir>` over an `n1x.yaml` renders a
project to try. The packages a web app it generates pins (next 16.3.2, react
and react-dom 19.2.8, TanStack Query, sonner, zod, Tailwind 4, TypeScript),
plus the `geist` package, are installed in `/opt/web-deps/node_modules`, and
the component preview's own packages in `/opt/preview-deps/node_modules`, if
you want to run or type-check rendered code or the preview without a network
install. With `NEXT_FONT_GOOGLE_MOCKED_RESPONSES=/opt/google-fonts/responses.cjs`
set, `next/font/google` serves Geist and Geist Mono from local files instead of
Google Fonts.