feat: keep every portaled component inside a shadow root

[antfubot]

Closes #30

Why

FormSelect (and every other popper, plus toasts) teleported to document.body, so inside a shadow root the content left the tree holding the stylesheet and rendered unstyled. Only the dialog family had been fixed in #29.

What changes

• All portaled components detect the enclosing shadow root and teleport into it, with a to?: string | HTMLElement override — FormSelect, FormCombobox, LayoutMenubar, OverlayDropdown, OverlayDropdownSub, OverlayHoverCard, OverlayContextMenu, FeedbackToasts, same as OverlayModal/OverlayDrawer/OverlayConfirm.
• usePortalTarget resolves the component's own root node instead of taking an anchor ref, so the display: contents wrapper divs the dialogs carried are gone and components whose root is a real element keep inherited attrs where they belong.
• Dark mode inside the portal container. dark: utilities compile to .dark .x; a container appended to the ShadowRoot had no .dark ancestor, so portaled content rendered light in a dark host. The container now mirrors the dark/light class of the nearest scheme wrapper and follows toggles.
• shimmer emits :where(.dark) instead of :where(html.dark), so it matches the same descendant convention as every other dark token.
• test/portal.test.ts opens every portaled component in a shadow root (and in the body) and covers the scheme mirroring; a new Utilities/ShadowRoot story mounts them all inside a shadow root for visual checks. README and AGENTS.md document the convention.

Verified

Light and dark, inside the story's shadow root — zero popups land in document.body:

Dropdown + submenu (dark)	Select (dark)	Modal + toast (dark)
menus in shadow: 2, in body: 0	listbox in shadow, container class dark	both in shadow, none in body

Not in scope: vue-afloat tooltips are already shadow-aware at the library level; the Utilities/* Overview links were already off (utility- vs utilities-) before this change.

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Created with the help of an agent.