docs(ui): note Last.fm API env vars on scrobbling tab

allstarr/wwwroot/index.html

@@ -518,7 +518,7 @@
             <div class="card">
                 <h2>Last.fm</h2>
                 <p style="color: var(--text-secondary); margin-bottom: 16px;">
-                    Scrobble to Last.fm. Enter your Last.fm username and password below, then click "Authenticate & Save" to generate a session key.
+                    Scrobble to Last.fm. Set <code>SCROBBLING_LASTFM_API_KEY</code> and <code>SCROBBLING_LASTFM_SHARED_SECRET</code> in your .env (from <a href="https://www.last.fm/api/account/create" target="_blank" rel="noopener" style="color: var(--accent);">last.fm/api/account/create</a>), restart, then enter username/password and click "Authenticate & Save".
                 </p>
 
                 <div class="config-section" style="margin-bottom: 24px;">