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/dashboard/build.mjs

@@ -74,9 +74,9 @@ await build({
   target: ['es2022'],
   sourcemap: true,
   logLevel: 'info',
-  // `@hive/shared/modal.js` imports its shadow-DOM component CSS
-  // (hive-dialog.css, hive-toast.css, component-common.css) as raw text
-  // via a plain `import css from './foo.css'` — the `text` loader turns
+  // `@hive/shared/modal.js` and `hive-btn.js` import their shadow-DOM
+  // component CSS (hive-dialog.css, hive-toast.css, hive-btn.css) as raw
+  // text via a plain `import css from './foo.css'` — the `text` loader turns
   // that into a string constant at bundle time instead of erroring on an
   // unrecognised extension. None of these JS entries import a `.css`
   // file any other way, so this doesn't collide with the separate