ui: cover art in Beatport, Spotify e YouTube search

- core/beatport.py: BeatportTrack.image_url estratto da image.dynamic_uri (95x95)
- core/spotify_client.py: _track_to_dict include image_url (Spotify torna 3 taglie,
  prendo la ~64px). search_artist_discography inietta album.images anche negli
  album-tracks per la cover
- core/youtube_search.py: image_url = i.ytimg.com/vi/<id>/mqdefault.jpg
- webui/index.html: nuova <th class="col-cover"></th> nelle 3 tabelle
- webui/js/app.js: 3 renderTable inseriscono <td class="col-cover"><img>
  con loading=lazy, referrerpolicy=no-referrer e onerror fallback
- webui/css/style.css: .beatport-table .col-cover img { 40x40, border-radius 4px,
  object-fit: cover, bg fallback var(--bg-input) }

webui/css/style.css

@@ -1667,6 +1667,15 @@ input[type="number"]::-webkit-inner-spin-button {
 }
 
 .beatport-table .col-check { width: 36px; }
+.beatport-table .col-cover { width: 52px; padding: 4px 8px; }
+.beatport-table .col-cover img {
+  width: 40px;
+  height: 40px;
+  border-radius: 4px;
+  object-fit: cover;
+  display: block;
+  background: var(--bg-input);
+}
 .beatport-table .col-pos   { width: 40px; color: var(--text-3); font-variant-numeric: tabular-nums; }
 .beatport-table .col-dur   { width: 70px; color: var(--text-2); font-variant-numeric: tabular-nums; text-align: right; }
 .beatport-table .col-state { width: 130px; font-size: 12px; }