fix: Zustand selector anti-pattern causing infinite re-render + plugin loader path

- Fix usePluginStore(s => s.getAll*()) selectors that returned new arrays
  on every call, causing infinite re-render loops and permanent spinner
- Affected: Sidebar, Settings, ContactDetail, ContactEditModal, PluginRouteRenderer
- Use usePluginStore(s => s.manifests) + useMemo instead
- Fix PluginRouteRenderer: show spinner instead of null when manifests loading
- Fix PluginLoader: @/ path replacement ../ -> ../../ for correct dynamic imports

frontend/src/components/layout/Sidebar.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useMemo } from 'react';
 import clsx from 'clsx';
 import { NavLink, useLocation } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
@@ -38,7 +38,13 @@ export function Sidebar() {
   const { t } = useTranslation();
   const { sidebarOpen, setSidebarOpen } = useUIStore();
   const location = useLocation();
-  const pluginMenuItems = usePluginStore(s => s.getAllMenuItems());
+  const manifests = usePluginStore(s => s.manifests);
+  const pluginMenuItems = useMemo(
+    () => manifests
+      .flatMap((m) => m.menu_items)
+      .sort((a, b) => a.order - b.order),
+    [manifests]
+  );
 
   const [expandedItems, setExpandedItems] = useState<Set<string>>(new Set());