Build the theme and contrast rows on the connected button group

`mode="picker"` hand-rolled an outlined segmented button, point for point the
design M3 Expressive deprecates, and its own arrow keys neither wrapped nor
handled Up/Down. Both rows are now <x-group>: native radios in a fieldset, so the
browser supplies the radiogroup, both axes, the wrap and the roving tab stop, and
the store is bound through an x-model accessor — a contrast row still marks the
resolved level but writes through setContrast(). A new `label` prop legends the
row. Plan: docs/plans/material-3-alignment.md, step 14 (navigation N-04, N-17).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/boost/skills/livewire-material-development/SKILL.md

@@ -821,7 +821,7 @@ An avatar that opens a menu: `name`, `email`, `avatar` (image URL or initials; d
 
 ### `<x-theme-toggle>`
 
-Switches `$store.theme`: `mode="toggle"` (default, light/dark icon button), `cycle` (light → dark → system), `picker` (a row of three for settings pages), `contrast` (the same row for M3's standard, medium and high levels, marking the one in force). Every toggle on a page shares the store.
+Switches `$store.theme`: `mode="toggle"` (default, light/dark icon button), `cycle` (light → dark → system), `picker` (a row of three for settings pages), `contrast` (the same row for M3's standard, medium and high levels, marking the one in force). Every toggle on a page shares the store. Both rows are `<x-group>` — a connected button group over native radios, so the arrow keys, the wrap and the roving tab stop are the browser's; `label` adds a visible legend, and without one the row is still named for a screen reader. Nothing is checked until Alpine has read the store, because the theme is known only in the browser.
 
 ### `<x-scheme-picker>`