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/index.html

@@ -891,6 +891,7 @@
               <thead>
                 <tr>
                   <th class="col-check"><input type="checkbox" id="beatport-select-all" /></th>
+                  <th class="col-cover"></th>
                   <th class="col-pos">#</th>
                   <th>Artista</th>
                   <th>Titolo (Mix)</th>
@@ -951,6 +952,7 @@
               <thead>
                 <tr>
                   <th class="col-check"><input type="checkbox" id="spotify-select-all" checked /></th>
+                  <th class="col-cover"></th>
                   <th class="col-pos">#</th>
                   <th>Artista</th>
                   <th>Titolo</th>
@@ -1008,6 +1010,7 @@
               <thead>
                 <tr>
                   <th class="col-check"><input type="checkbox" id="youtube-select-all" checked /></th>
+                  <th class="col-cover"></th>
                   <th class="col-pos">#</th>
                   <th>Titolo video</th>
                   <th>Canale</th>