Portal: Add PortalProvider for default container context

[rxliuli]

Summary

Add PortalProvider component to @radix-ui/react-portal, allowing users to set a default portal container via React context. This is particularly useful for browser extensions and Shadow DOM scenarios, where portaling to document.body breaks style isolation.

Without PortalProvider, every portal-based component (Dialog, Popover, Tooltip, Select, Menu, etc.) needs an explicit container prop — tedious and error-prone. Now users can set it once at the top level:

function App() {
  const [container, setContainer] = React.useState<HTMLElement | null>(null);
  return (
    <Portal.PortalProvider container={container!}>
      <div ref={setContainer} />
      <Dialog.Root>
        <Dialog.Trigger>Open</Dialog.Trigger>
        <Dialog.Portal>  {/* no container prop needed */}
          <Dialog.Content>...</Dialog.Content>
        </Dialog.Portal>
      </Dialog.Root>
    </Portal.PortalProvider>
  );
}

Container priority: explicit container prop > PortalProvider context > document.body

No changes needed to downstream components — they all pass container through to Portal, so undefined now falls back to context automatically.

Related issues

• Closes [Feat:Portal] Override default portal'd location  #2845 — [Feat:Portal] Override default portal'd location
• Related [feat]: Add Shadow DOM Support to React Components shadcn-ui/ui#7936 — [feat]: Add Shadow DOM Support to React Components

Test plan

• pnpm typecheck passes
• pnpm build passes
• pnpm lint passes
• Verify Portal still renders to document.body by default (no PortalProvider)
• Verify PortalProvider sets the default container for nested Portals
• Verify explicit container prop on Portal overrides PortalProvider

🤖 Generated with Claude Code

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Latest commit: a66b0e5

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