feat(quick-261022-imo): settings section panel scoped to the current category

- settingsSectionOf returns the current entry's category group (null for one-entry categories)
- SettingsPanel mirrors the plugin side menu, collapse included
- AppShell shows it on settings forms and settings-linked controllers without own navigation
- docs/backend/settings.md describes moving between settings pages

admin/src/state/useSettings.ts

@@ -64,6 +64,39 @@ export const settingsGroups = computed(() => {
   return groups
 })
 
-export function useSettings() {
-  return { settings, hasSettings, settingsGroups, loadSettings, settingsEntry, settingsPath, settingsLinkFor }
+/** The settings pages of one category, with the page on screen. */
+export interface SettingsSection {
+  category: string
+  current: SettingsEntry
+  entries: SettingsEntry[]
+}
+
+/**
+ * The category group of the given entry for the shell's settings panel, in
+ * settingsGroups order and without entries of any other category. Null when
+ * there is no current entry or its category holds a single page: there is
+ * nothing to switch to.
+ */
+export function settingsSectionOf(current: SettingsEntry | null): SettingsSection | null {
+  if (!current) {
+    return null
+  }
+  const group = settingsGroups.value.find((item) => item.category === current.category)
+  if (!group || group.entries.length < 2) {
+    return null
+  }
+  return { category: group.category, current, entries: group.entries }
+}
+
+export function useSettings() {
+  return {
+    settings,
+    hasSettings,
+    settingsGroups,
+    loadSettings,
+    settingsEntry,
+    settingsPath,
+    settingsLinkFor,
+    settingsSectionOf,
+  }
 }