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/rail/PanelCollapseButton.vue

@@ -0,0 +1,66 @@
+<script setup>
+import { storeToRefs } from 'pinia';
+import { computed } from 'vue';
+import { msg, t } from '../../i18n/index.js';
+import UiButton from '../kit/UiButton.vue';
+import { usePanelChromeStore } from '../stores/panelChrome.js';
+
+// A panel's collapse button: its glyph, aria-expanded and words follow the
+// panel's state in the panel chrome store, and a click asks PanelChrome to
+// toggle it. Until PanelChrome presents the panel it shows its markup's glyph
+// and tip.
+const props = defineProps({
+  target: { type: String, required: true },
+  /** The panel's name in "Expand {name}", already translated. */
+  name: { type: String, default: () => t('panel') },
+  /** A right-rail panel opens and closes sideways: ◀ / ▶, not + / −. */
+  sideways: { type: Boolean, default: false },
+  /** The glyph and tip before the panel is presented. */
+  initial: { type: String, required: true },
+  initialTitle: { type: String, default: () => t('Collapse panel') },
+  /** Words that replace "Expand/Collapse {name}" (Radio's own wording). */
+  words: { type: Object, default: null },
+});
+
+const chrome = usePanelChromeStore();
+const { panels } = storeToRefs(chrome);
+const state = computed(() => panels.value[props.target]);
+
+const glyph = computed(() => {
+  if (!state.value) return props.initial;
+  if (props.sideways) return state.value.collapsed ? '◀' : '▶';
+  return state.value.collapsed ? '+' : '−';
+});
+const label = computed(() => {
+  if (!state.value) return props.words?.initialLabel;
+  if (props.words)
+    return state.value.collapsed
+      ? props.words.expandLabel
+      : props.words.collapseLabel;
+  const key = state.value.collapsed
+    ? msg('Expand {name}')
+    : msg('Collapse {name}');
+  return t(key, { name: props.name });
+});
+const title = computed(() => {
+  if (!state.value) return props.initialTitle;
+  if (props.words)
+    return state.value.collapsed
+      ? props.words.expandTitle
+      : props.words.collapseTitle;
+  return label.value;
+});
+</script>
+
+<template>
+  <UiButton
+    variant="icon"
+    class="panel-collapse-btn"
+    :data-collapse-target="target"
+    :label="label"
+    :title="title"
+    :aria-expanded="state ? String(!state.collapsed) : undefined"
+    @click="chrome.act('toggle', target)"
+    >{{ glyph }}</UiButton
+  >
+</template>