fix(quick-261016-0fm): bottom save bars no longer cover the sidebar

AppShell exposes --shell-offset (the rail, plus the section panel when it
shows); the form, settings, My account and preview save bars start there,
so the rail's Settings item stays visible and clickable.

admin/src/components/shell/AppShell.vue

@@ -27,10 +27,17 @@ onBeforeUnmount(stopViewport)
 const siteURL = `${window.location.origin}/`
 
 const active = computed(() => activeEntry(String(route.params.vendor ?? ''), String(route.params.plugin ?? '')))
+
+// --shell-offset is the width the sidebar occupies (the rail, plus the
+// section panel when it shows). Fixed bottom save bars start there, so the
+// rail's Settings item is never covered.
+const shellOffset = computed(() =>
+  active.value && !collapsed.value ? 'calc(var(--spacing-rail) + var(--spacing-panel))' : 'var(--spacing-rail)',
+)
 </script>
 
 <template>
-  <div class="flex min-h-screen bg-bg text-text">
+  <div class="flex min-h-screen bg-bg text-text" :style="{ '--shell-offset': shellOffset }">
     <PluginRail />
     <SectionPanel v-if="active && !collapsed" :entry="active" />
     <div class="flex min-w-0 flex-1 flex-col">