frontend/shared: add attachShadowCss, the shared shadow-DOM style-attach helper

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.

frontend/packages/shared/package.json

@@ -17,7 +17,8 @@
     "./chrome.css": "./src/chrome.css",
     "./forms.js": "./src/forms.js",
     "./dom.js": "./src/dom.js",
-    "./modal.js": "./src/modal.js"
+    "./modal.js": "./src/modal.js",
+    "./shadow-css.js": "./src/shadow-css.js"
   },
   "files": [
     "src/"