feat(webkit): give Menu rows an arrow control split by href, and make Back a button

[gabriel-lisboa-azion]

Summary

• A row that owns children now always carries a trailing transparent IconButton arrow — the affordance that says the row owns children. href decides what the label beside it does: with one, the label is a real <a> and the arrow alone reveals the children; without one, the label reveals them too, so the whole row does.
• aria-expanded / aria-controls sit on whichever control expands the children and on exactly one of them; when the label owns them the arrow becomes a redundant pointer target (tabindex="-1" + aria-hidden), adding neither a second tab stop nor a second announced control.
• Menu.Back is a compact button instead of a full-width row with a centred label, and its text names where it lands (Back, Back to Settings, or label) rather than the level the reader is already in.

How to test

1. pnpm webkit:test — 2216 pass, including the menu suite's axe checks.
2. pnpm storybook:dev → Components / Navigation / Menu / Types. Neither row has an href, so each reveals its children from the whole row while still showing the arrow. Tab: one stop per row.
3. Add href="/anything" to a SubTrigger in that story: the label becomes a real link (middle-click opens a tab), the arrow alone reveals, and aria-expanded moves from the label to the arrow.
4. Menu / Drill — activate the row to push its level; Back appears as a compact left-aligned button reading Back. Push a second level: Back to <parent>. Escape / ArrowLeft / clicking Back pops and returns focus to whichever control pushed.
5. Menu / Disabled — the disabled trigger suppresses every control of the row, and its arrow paints no fill (IconButton's filled disabled box would otherwise make the arrow the brightest thing on a row whose label had dimmed away).

Notes

• Not marked breaking: no prop, event, slot or export was removed or renamed, and a row with no href behaves as it did before this PR. The trigger's root is now always a div holding its controls, so a consumer whose tests or CSS target the trigger root as the interactive element should point at …__control / …__reference / …__arrow.
• New prop: SubTrigger.href (default ''). In data-driven mode it comes from MenuNode.href, which already existed — so the behaviour falls out of the tree with no new concept for consumers, and it follows root-element.md (the element switches on a data prop, never an as string).
• IconButton now forwards $attrs to its root, per .claude/rules/root-element.md. It declared inheritAttrs: false and never spread them, so every consumer attribute was silently swallowed — that is why the arrow could not carry aria-expanded. class and data-testid stay excluded (already applied explicitly).
• The unconditional split was wrong on a phone, measured in the sample's docs sheet: the label was 320×32 and navigated away and dismissed the sheet, while the only control that expanded was 28×28 — 7% of the row's area, under the 44px touch guideline.