frontend: one component = one dir for hive-btn/hive-dialog/hive-toast

Splits the shadow-DOM custom elements out of the flat shared/src layout
into per-component directories:

  hive-btn/hive-btn.{js,css}
  hive-dialog/hive-dialog.{js,css}
  hive-toast/hive-toast.{js,css}

hive-dialog and hive-toast were previously defined inline inside
modal.js alongside the openDialog/themedConfirm/themedPrompt/themedToast
orchestration helpers; modal.js is now a slim entry point that imports
the two component modules for their customElements.define side effect
and keeps only the orchestration functions, which aren't components
themselves. hive-dialog.js now imports hive-btn.js directly (it's the
actual consumer that creates <hive-btn> elements), instead of modal.js
importing it on hive-dialog's behalf.

Pulled the identical shadow-root-plus-adopted-stylesheet boilerplate
(previously duplicated between modal.js's local attachShadow() and
hive-btn.js's inline version) into a shared shadow-css.js helper,
attachShadowCss(host, cssText, shadowInit), used by all three
components. Behaviorally identical — same attachShadow() options per
component, just deduplicated.

No external import paths changed: every consumer only ever imported
the package-level @hive/shared/modal.js entry point, never the
component internals directly, so this is fully internal to the shared
package. Verified with a full frontend build (dashboard + agent
bundles).

frontend/packages/shared/src/hive-btn/hive-btn.css

@@ -0,0 +1,46 @@
+/* hive-btn.css — shadow-DOM stylesheet for the <hive-btn> autonomous
+   custom element (hive-btn.js). Loaded as raw text at build time
+   (esbuild's `text` loader) and adopted into each instance's own shadow
+   root. `:host` is an invisible wrapper (`display: contents`) — the
+   real box is the plain `button` selector below, targeting the actual
+   `<button>` the shadow root wraps (scope-isolated by the shadow
+   boundary already, no clash risk with anything outside). `variant` is
+   an attribute on the HOST (how callers set it, and where hive-dialog's
+   own selectors read it back); `:host([variant="…"])` sets `color`,
+   which then inherits down into the shadow tree the normal way. */
+
+:host {
+  display: contents;
+}
+:host([variant="cancel"]) { color: var(--subtext0); }
+:host([variant="confirm"]) { color: var(--green); }
+:host([variant="danger"]) { color: var(--red); }
+
+button {
+  font-family: inherit;
+  font-size: inherit;
+  font-weight: bold;
+  text-transform: uppercase;
+  letter-spacing: 0.1em;
+  background: transparent;
+  -webkit-appearance: none;
+  appearance: none;
+  border: 1px solid;
+  padding: 0.25em 0.8em;
+  cursor: pointer;
+  color: inherit;
+  text-shadow: 0 0 4px currentColor;
+  box-shadow: 0 0 0 0 currentColor;
+  transition: box-shadow 0.15s ease;
+}
+button:hover {
+  background: color-mix(in srgb, var(--fg) 6%, transparent);
+  text-shadow: 0 0 10px currentColor;
+  box-shadow: 0 0 10px -2px currentColor;
+}
+button:disabled {
+  opacity: 0.32;
+  cursor: not-allowed;
+  text-shadow: none;
+  box-shadow: none;
+}

frontend/packages/shared/src/hive-btn/hive-btn.js

@@ -0,0 +1,56 @@
+// hive-btn.js — <hive-btn>, the button primitive for modal.js's dialog
+// buttons. Autonomous custom element (previously a customized built-in,
+// `<button is="hive-btn">` — dropped: `is=` upgrades can't host a shadow
+// root at all, which crashed every themed dialog, and reads as a hack
+// regardless once you know that). A real `<button>` lives inside the
+// shadow root instead, so native click/keyboard activation and
+// `:disabled` still come for free — just one level down from the host.
+//
+// `delegatesFocus: true` on the shadow root means `.focus()` on the host
+// (what modal.js calls) focuses the inner button directly, and a click
+// anywhere on the host focuses it too, matching native `<button>` feel.
+// The inner button's native `click` is a composed event, so it bubbles
+// out through the shadow boundary — callers add `click` listeners on the
+// `<hive-btn>` host exactly as they would on a plain `<button>`.
+//
+// Usage: `el('hive-btn', { type: 'button', variant: 'danger' }, 'label')`.
+// `variant` ∈ 'cancel' | 'confirm' | 'danger' | unset (neutral default) —
+// reflected straight through to the inner button so hive-btn.css's
+// `:host([variant="…"])` selectors keep working unchanged.
+// `disabled`/`type` are likewise plain attributes on the host, mirrored
+// onto the inner button on connect and on every attribute change.
+
+import { attachShadowCss } from '../shadow-css.js';
+import hiveBtnCss from './hive-btn.css';
+
+const OBSERVED = ['disabled', 'type'];
+
+class HiveBtn extends HTMLElement {
+  static get observedAttributes() {
+    return OBSERVED;
+  }
+
+  connectedCallback() {
+    if (this._btn) {
+      this._sync();
+      return; // already built (e.g. re-parenting re-fires connectedCallback)
+    }
+    const root = attachShadowCss(this, hiveBtnCss, { delegatesFocus: true });
+    const btn = document.createElement('button');
+    btn.append(document.createElement('slot'));
+    root.append(btn);
+    this._btn = btn;
+    this._sync();
+  }
+
+  attributeChangedCallback() {
+    this._sync();
+  }
+
+  _sync() {
+    if (!this._btn) return;
+    this._btn.disabled = this.hasAttribute('disabled');
+    this._btn.type = this.getAttribute('type') || 'button';
+  }
+}
+customElements.define('hive-btn', HiveBtn);