fix(ui): always show the Last.fm link on the artist details page

The button only rendered when an agent supplied a real last.fm URL, either
embedded in the biography or as artistInfo.lastFmUrl. Neither source is
reliable anymore: cleanContent strips the "Read more on Last.fm" anchor out of
the biography, and the Last.fm agent does not register at all unless
LastFM.ApiKey and LastFM.Secret are set, in which case GetArtistURL falls
through to ListenBrainz, which returns the artist's official homepage. The
isLastFmURL guard then correctly rejects it and the button disappears.

Build the URL from the artist name when no canonical one is available, the same
way AlbumExternalLinks already does for albums. A real last.fm URL is still
preferred when one is present, and the button stays hidden when Last.fm is
disabled or the artist has no name.

ui/src/artist/ArtistExternalLink.jsx

@@ -38,15 +38,15 @@ const ArtistExternalLinks = ({ artistInfo, record }) => {
   }
 
   if (config.lastFMEnabled) {
-    if (lastFMlink && isLastFmURL(lastFMlink[2])) {
+    // No agent may be enabled to supply a canonical URL, so fall back to a name-based one.
+    const lastFMUrl =
+      (lastFMlink && isLastFmURL(lastFMlink[2]) && lastFMlink[2]) ||
+      (isLastFmURL(artistInfo?.lastFmUrl) && artistInfo.lastFmUrl) ||
+      (record.name &&
+        `https://last.fm/music/${encodeURIComponent(record.name)}`)
+    if (lastFMUrl) {
       addLink(
-        lastFMlink[2],
-        'message.openIn.lastfm',
-        <ImLastfm2 className="lastfm-icon" />,
-      )
-    } else if (isLastFmURL(artistInfo?.lastFmUrl)) {
-      addLink(
-        artistInfo?.lastFmUrl,
+        lastFMUrl,
         'message.openIn.lastfm',
         <ImLastfm2 className="lastfm-icon" />,
       )

ui/src/artist/ArtistExternalLink.test.jsx

@@ -0,0 +1,60 @@
+import React from 'react'
+import { render, screen } from '@testing-library/react'
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { TestContext } from 'ra-test'
+import ArtistExternalLinks from './ArtistExternalLink'
+
+const { mockConfig } = vi.hoisted(() => ({
+  mockConfig: { lastFMEnabled: true },
+}))
+vi.mock('../config', () => ({ default: mockConfig }))
+
+describe('ArtistExternalLinks', () => {
+  beforeEach(() => {
+    mockConfig.lastFMEnabled = true
+  })
+
+  const renderLinks = (artistInfo, record = { id: 'ar-1', name: 'Björk' }) =>
+    render(
+      <TestContext>
+        <ArtistExternalLinks artistInfo={artistInfo} record={record} />
+      </TestContext>,
+    )
+
+  const lastFmHref = () =>
+    screen.getByLabelText('message.openIn.lastfm').closest('a').href
+
+  it('uses the URL returned by the server', () => {
+    renderLinks({ lastFmUrl: 'https://www.last.fm/music/Bjork' })
+    expect(lastFmHref()).toBe('https://www.last.fm/music/Bjork')
+  })
+
+  it('uses the URL found in the biography', () => {
+    renderLinks({
+      biography: 'Read more on <a href="https://www.last.fm/music/Bjork">',
+      lastFmUrl: 'https://bjork.com',
+    })
+    expect(lastFmHref()).toBe('https://www.last.fm/music/Bjork')
+  })
+
+  it('builds the URL from the artist name when the server has none', () => {
+    renderLinks({ lastFmUrl: 'https://bjork.com' })
+    expect(lastFmHref()).toBe('https://last.fm/music/Bj%C3%B6rk')
+  })
+
+  it('builds the URL when there is no artist info at all', () => {
+    renderLinks(undefined)
+    expect(lastFmHref()).toBe('https://last.fm/music/Bj%C3%B6rk')
+  })
+
+  it('shows no Last.fm link when Last.fm is disabled', () => {
+    mockConfig.lastFMEnabled = false
+    renderLinks({ lastFmUrl: 'https://www.last.fm/music/Bjork' })
+    expect(screen.queryByLabelText('message.openIn.lastfm')).toBeNull()
+  })
+
+  it('shows no Last.fm link when the artist has no name', () => {
+    renderLinks({}, { id: 'ar-1', name: '' })
+    expect(screen.queryByLabelText('message.openIn.lastfm')).toBeNull()
+  })
+})