feat(panels): render collapse, dock and pin buttons from the store

PanelChrome no longer writes the panels' collapse buttons, the command
dock tray toggles' names or the dock pins' pressed state. It presents each
panel's open and pinned state to the new gev-panel-chrome store, and the
buttons, now views, call it there: PanelCollapseButton for the panel
headers, bound toggles and pins in the command dock. Clicks take effect
once the chrome is initialized; a new owner starts from its own states.

bindPanelDisclosure binds only a panel's Escape and refuses the retired
buttons and onChange arguments. UiButton joins the component kit.

src/ui/kit/UiButton.vue

@@ -0,0 +1,47 @@
+<script setup>
+// Core's button, with @halfpixel/ui-vue UiButton's props, classes and
+// attribute contract so the two kits can merge: the button is always the root,
+// attributes passed in win over its own (a `title` replaces the label's), and
+// `pressed` renders aria-pressed only when it is passed. Core styles the
+// button through the classes its callers pass; the kit classes carry no Core
+// styles yet. Not mirrored yet: icon, loading, disabledReason and the
+// missing-label warning.
+defineOptions({ inheritAttrs: false });
+
+const props = defineProps({
+  variant: { type: String, default: 'ghost' },
+  tone: { type: String, default: 'accent' },
+  /** The accessible name (aria-label) and, with nativeTitle, the tip. */
+  label: { type: String, default: undefined },
+  pressed: { type: Boolean, default: undefined },
+  disabled: { type: Boolean, default: false },
+  type: { type: String, default: 'button' },
+  /** The id of the form a submit button belongs to, where it sits outside it. */
+  form: { type: String, default: undefined },
+  nativeTitle: { type: Boolean, default: true },
+});
+const emit = defineEmits(['click']);
+</script>
+
+<template>
+  <button
+    :type="props.type"
+    :form="props.form"
+    class="ui-button ui-focus"
+    :class="[
+      `ui-button--${props.variant}`,
+      `ui-button--${props.tone}`,
+      { 'ui-button--pressed': props.pressed },
+    ]"
+    :aria-label="props.label"
+    :title="props.nativeTitle ? props.label : undefined"
+    :aria-pressed="
+      props.pressed === undefined ? undefined : props.pressed ? 'true' : 'false'
+    "
+    :disabled="props.disabled"
+    v-bind="$attrs"
+    @click="emit('click', $event)"
+  >
+    <slot />
+  </button>
+</template>