fix(components): forward data-slot to component root

[benjamincanac]

🔗 Linked issue

Follow-up to #6641. That PR forwarded fall-through attributes to Separator's root (the inconsistency was surfaced by #6611), but its data-slot-on-root handling was reverted before merge, so the static data-slot="root" still won over a caller value. This re-applies it on Separator and generalizes the same rule to every component.

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Components forward data-slot to their children in roughly 80 places (most often to UIcon, UAvatar, UButton, UInput), but whether the child honored that data-slot on its own root was inconsistent. Single root components with default inheritAttrs let the caller's value fall through and win, which is what we want. Components with inheritAttrs: false did not: some dropped it from the root, and some let it leak onto an inner element. UAvatar is the clearest case, it placed the caller's data-slot on the inner <img> instead of the root, so a parent's [data-slot="leadingAvatar"] never matched where it expected.

This makes the behavior consistent everywhere. The canonical rule is: a caller-supplied data-slot always wins on the component's root, with the component's own value (root/base) as the fallback, and inner elements keep their own data-slot.

Three idioms depending on how the root receives attributes:

1. Single root, default inheritAttrs: nothing to do, Vue fallthrough already handles it.
2. inheritAttrs: false with $attrs on the root: put the default before the spread, either data-slot="root" v-bind="$attrs" (bare) or v-bind="{ 'data-slot': 'root', ...$attrs }" (object), so a caller value overrides it.
3. inheritAttrs: false with $attrs forwarded to an inner element (UAvatar, UInput, UCheckbox, ...): read the value on the root with :data-slot="($attrs['data-slot'] as string | undefined) ?? 'root'", and keep each inner element's own data-slot after its $attrs spread (or strip it with { ...$attrs, 'data-slot': undefined } for <Slot> forwards and unlabeled inputs) so the caller's value does not leak.

No component's own data-slot value changes by default (roots stay root / base). The snapshot churn is the intended behavior surfacing where parents pass data-slot to embedded avatars and inputs, plus some benign attribute reordering on the roots where data-slot now sits before the $attrs spread. No public API changed. The convention is documented in component-structure.md and AGENTS.md.

EditorToolbar was intentionally left as is, its $attrs lands on a conditional :as="Component" / template outer element and it is never passed a data-slot.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@6643

commit: 17ef442