Pulls the attachShadow-plus-plain-<style>-tag boilerplate that hive-dialog
and hive-toast already build per component into one shared helper, so the
next shadow-DOM custom element (the agent context menu, next commit) can
reuse it instead of re-deriving the same few lines. A plain <style>
element rather than a constructed CSSStyleSheet with adoptedStyleSheets --
each component instance builds its own fresh stylesheet per connect with
no sharing across instances, so adoptedStyleSheets buys nothing here over
the plain, universally-understood <style> tag.
ed92883294