refactor(sidebar): co-locate sidebar feature into features/sidebar

src/features/sidebar/index.ts

@@ -0,0 +1,9 @@
+/**
+ * Sidebar feature — the primary nav sidebar (library/system nav, playlists
+ * section, library picker, active jobs) plus its nav config store, reorder
+ * helpers, and the in-sidebar performance-probe overlay (Ctrl+Shift+P).
+ */
+export { default } from './components/Sidebar';
+export { useSidebarStore, CONSERVED_SIDEBAR_NAV_IDS } from './store/sidebarStore';
+export type { SidebarItemConfig } from './store/sidebarStore';
+export { applySidebarReorderById, resolveStartRoute } from './utils/sidebarNavReorder';