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/pages/Settings.tsx

@@ -1,4 +1,4 @@
-import React from 'react';
+import React, { useMemo } from 'react';
 import { NavLink, Outlet } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
 import { usePluginStore } from '@/store/pluginStore';
@@ -6,7 +6,13 @@ import * as LucideIcons from 'lucide-react';
 
 export function SettingsPage() {
   const { t } = useTranslation();
-  const pluginSettingsPages = usePluginStore(s => s.getAllSettingsPages());
+  const manifests = usePluginStore(s => s.manifests);
+  const pluginSettingsPages = useMemo(
+    () => manifests
+      .flatMap((m) => m.settings_pages)
+      .sort((a, b) => a.order - b.order),
+    [manifests]
+  );
 
   const hardcodedNavItems = [
     { to: '/settings/system', label: t('systemSettings.title'), icon: '\u2699\ufe0f' },