refactor(docs): consume layer plumbing instead of forking it - #330

`@uxfront/layer-docs` now ships the collections builder options, the
locale files, the Content Studio schema and the Vite prebundle hints, so
the local forks are dead weight. Net -2150 lines.

- `content.config.ts`: 118 lines -> 11. `defineDocsCollections(DOCS_SECTIONS,
  { sitemap: true, changelog: true })`. Section topology stays local in
  `app/constants/sections.ts`; the collection shapes, per-locale fan-out and
  schemas come from the layer.
- Deleted `i18n/locales/` (31 files), `nuxt.schema.ts`,
  `modules/optimizeDeps.ts` and `modules/useDocusI18nOverride.ts`. That
  module's own docblock said to remove it once the package shipped its
  locales; it does now.
- Deleted 18 shadow copies that re-declared layer files at the same relative
  path. Layer precedence meant every upstream fix to those files was
  invisible here — notably the `AppHeader` anchor-nesting a11y fix, which is
  now actually in effect (verified: max anchor nesting depth 1 in the
  prerendered header).
- Deleted `utils/prerender.ts`; the layer's `app/utils/prerender.ts` is
  equivalent and auto-imported, so the explicit relative import goes too.
- `app.config.ts` declares `docsTheme.frameworks` (React/Vanilla/Vue, order
  preserved). The layer no longer ships a default list.

Four shadows are kept deliberately, each now carrying a docblock naming its
delta and its deletion trigger:

- `useDocsSections.ts` — the `THEME_SUBSECTIONS` split is a styleframe
  content shape. Rebased onto the layer copy, which adds the missing
  `hasSectionSwitcher`; without it the layer's `AppSubHeader` read
  `undefined` and the docs sub-header never rendered.
- `DocsAsideLeftBody.vue` — theme-subsection nav filter and the `ProIcon`
  badge.
- both `[...slug].vue` pages — `defineOgImage`; `nuxt-og-image` and the
  `DocsSatori` template are registered here, not in the theme. Rebased to
  take the layer's `USeparator decorative` a11y fix.
- `app/types/non-route-categories.ts` — a deliberate duplicate, because
  `modules/nonRouteCategories.ts` imports it by relative path at build time
  and that module has not moved into the layer yet.

Requires @uxfront/layer-docs@0.2.0, which is not published yet.

Refs UXF-119, UXF-3

It imported `~~/i18n/locales/en.json`, which moved into
`@uxfront/layer-docs` with the rest of the locale files, and it had zero
call sites — the layer's `useDocusI18n` is what the components actually
use. Its docblock also referenced `apps/shared/modules/config.ts`, a path
that does not exist in this repo, so it had been dead for a while.

The pin moved to ^0.2.0 with the layer extraction, but the lockfile still
resolved 0.1.0 because that release did not exist yet. It does now, so
the entry is regenerated against the registry — no local override.