frontend: add <hive-btn> component, use it for dialog buttons

mara: 'wait the common styles is literally just the button stuff? pls
make a button component now as part of this pr and replace the usage
in the modal. replacing all usages and finally removing the btn styles
from common css is a follow up then.'

<hive-btn> (hive-btn.js) is a customized built-in <button is="hive-btn">
with its own shadow root -- extending HTMLButtonElement keeps every
native button behaviour (click/keyboard activation, :disabled, form
participation) instead of re-implementing it on a generic wrapper.
Shadow root holds only an adopted stylesheet + a <slot>, so the
button's light-DOM content (label, or asyncBtn's swapped-in spinner
span) renders through unchanged -- slotted content stays styled by the
light-DOM cascade, so the global .spinner class still applies.
Variants (cancel/confirm/danger) are a 'variant' attribute, not a CSS
class, since they're a semantic prop of the component.

Customized built-ins aren't supported in Safari/WebKit -- fine here,
the project targets recent Firefox only (same reasoning as the
original custom-elements pilot).

Wired into modal.js: HiveDialog's buttons now render as
<button is="hive-btn" variant="...">, replacing the old
component-common.css .btn copy -- deleted that file + component-
styles.js entirely (their sole purpose was giving dialog buttons a
.btn look, which hive-btn now owns properly). dom.js's el() gained
 support so it can create customized built-ins the same way it
creates everything else. hive-dialog.css dropped the now-dead
.cancel/.confirm/.confirm.danger rules.

Per mara's scoping: NOT touching the other .btn consumers across the
app (dashboard/agent submit buttons, form() helper, etc.) or removing
.btn from dashboard/common.css / agent/agent.css in this PR -- that
migration + cleanup is an explicit follow-up.

Verified with a full frontend build (grepped bundled JS for hive-btn/
variant to confirm it inlines); nix fmt clean.

frontend/packages/shared/package.json

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