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/contacts/ContactDetail.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useMemo } from 'react';
 import { useTranslation } from 'react-i18next';
 import { Button } from '@/components/ui/Button';
 import { Badge } from '@/components/ui/Badge';
@@ -157,7 +157,14 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
   const [personModalOpen, setPersonModalOpen] = useState(false);
   const [editingPerson, setEditingPerson] = useState<ContactPerson | null>(null);
   const [activeTab, setActiveTab] = useState('details');
-  const pluginTabs = usePluginStore(s => s.getDetailTabsForEntity('contact'));
+  const manifests = usePluginStore(s => s.manifests);
+  const pluginTabs = useMemo(
+    () => manifests
+      .flatMap((m) => m.detail_tabs)
+      .filter((t) => t.entity_type === 'contact')
+      .sort((a, b) => a.order - b.order),
+    [manifests]
+  );
   const aiActiveTab = useAIUIControlStore(s => s.activeTab);
   const aiActiveModal = useAIUIControlStore(s => s.activeModal);
 
@@ -180,7 +187,12 @@ export function ContactDetail({ contact, loading, onEdit, onDeleted }: ContactDe
   const user = useAuthStore(s => s.user);
 
   // Custom fields from plugin definitions
-  const customFieldDefs = usePluginStore(s => s.getCustomFieldsForEntity('contact'));
+  const customFieldDefs = useMemo(
+    () => manifests
+      .flatMap((m) => m.custom_fields || [])
+      .filter((cf) => cf.entity === 'contact'),
+    [manifests]
+  );
   const { data: customFieldsData } = useCustomFields(contact?.id);
 
   if (loading) {