feat(plugins): surface the valid agent names in logs and the Plugins UI (#5910)

The agent name used in the `Agents` config option comes from the .ndp file
name, not from the manifest. The Plugins UI showed the ID but never said what
it was for, so renaming a plugin file silently breaks the config with only a
Debug-level "Unknown agent ignored" line to go on.

Add a caption under the ID in the Plugins UI, and list the accepted names
alongside the rejected one in that log line.

Related to navidrome/apple-music-plugin#14

ui/src/plugin/InfoCard.jsx

@@ -123,6 +123,13 @@ export const InfoCard = ({ record, manifest, classes, translate, isSmall }) => (
           isSmall={isSmall}
         >
           {record.id}
+          <Typography
+            variant="caption"
+            color="textSecondary"
+            className={classes.fieldHelp}
+          >
+            {translate('resources.plugin.messages.idHelp')}
+          </Typography>
         </InfoRow>
 
         {manifest?.name && (
@@ -201,7 +208,7 @@ export const InfoCard = ({ record, manifest, classes, translate, isSmall }) => (
               <Typography
                 variant="caption"
                 color="textSecondary"
-                style={{ marginTop: 4, display: 'block' }}
+                className={classes.fieldHelp}
               >
                 {translate('resources.plugin.messages.clickPermissions')}
               </Typography>

ui/src/plugin/InfoCard.test.jsx

@@ -0,0 +1,41 @@
+import React from 'react'
+import { render, screen } from '@testing-library/react'
+import { describe, it, expect, vi } from 'vitest'
+
+vi.mock('../common', () => ({
+  DateField: ({ source }) => <span data-testid={`date-${source}`} />,
+}))
+
+const { InfoCard } = await import('./InfoCard')
+
+const record = {
+  id: 'apple-music',
+  path: '/data/plugins/apple-music.ndp',
+  updatedAt: '2026-01-01T00:00:00Z',
+  createdAt: '2026-01-01T00:00:00Z',
+}
+
+const renderCard = () =>
+  render(
+    <InfoCard
+      record={record}
+      manifest={{ name: 'Apple Music Metadata Agent' }}
+      classes={{}}
+      translate={(key) => key}
+      isSmall={false}
+    />,
+  )
+
+describe('InfoCard', () => {
+  it('shows the plugin ID', () => {
+    renderCard()
+    expect(screen.getByText('apple-music')).toBeInTheDocument()
+  })
+
+  it('explains that the ID is the name used in config options', () => {
+    renderCard()
+    expect(
+      screen.getByText('resources.plugin.messages.idHelp'),
+    ).toBeInTheDocument()
+  })
+})

ui/src/plugin/styles.js

@@ -45,6 +45,10 @@ export const usePluginShowStyles = makeStyles(
       fontSize: '0.85rem',
       wordBreak: 'break-all',
     },
+    fieldHelp: {
+      marginTop: theme.spacing(0.5),
+      display: 'block',
+    },
     permissionsContainer: {
       display: 'flex',
       flexWrap: 'wrap',