fix: preserve scoped themes in shadcn portals

[fzdwx]

Shadcn dialogs, menus, popovers, selects and tooltips currently portal to document.body, so a renderer inside a scoped theme loses its font and theme variables when an overlay opens.

Add PortalContainerProvider to forward the nearest host-owned element to the existing Radix portals. Without a provider, the default body container remains unchanged. Component schemas, bindings, state and actions are unchanged. Package docs, API docs and the integration skill describe ownership and clipping considerations.

Validation:

• Rebuilt the exact v0.21.0 package with these source changes: ESM, CJS and declaration builds passed.
• Downstream local Chrome: 36 component and interaction checks passed, including nested dialog keyboard containment.
• Light/dark App, public share and fully offline file export portal checks passed.
• Full workspace type-check was not completed: pnpm attempted to install the entire workspace in the isolated checkout; the install was stopped. No workspace-wide type-check pass is claimed.

[vercel]

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