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; }

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>

webui/js/app.js

@@ -1297,6 +1297,19 @@ const BeatportUI = (function () {
       tdCheck.appendChild(cb);
       tr.appendChild(tdCheck);
 
+      const tdCover = document.createElement("td");
+      tdCover.className = "col-cover";
+      if (t.image_url) {
+        const img = document.createElement("img");
+        img.src = t.image_url;
+        img.alt = "";
+        img.loading = "lazy";
+        img.referrerPolicy = "no-referrer";
+        img.onerror = () => { img.style.visibility = "hidden"; };
+        tdCover.appendChild(img);
+      }
+      tr.appendChild(tdCover);
+
       const tdPos = document.createElement("td");
       tdPos.className = "col-pos";
       tdPos.textContent = String(t.position || (i + 1));
@@ -1518,8 +1531,12 @@ const SpotifyUI = (() => {
       const tr = document.createElement("tr");
       tr.dataset.idx = String(i);
       if (already) tr.classList.add("already-downloaded");
+      const coverImg = t.image_url
+        ? `<img src="${_escape(t.image_url)}" alt="" loading="lazy" referrerpolicy="no-referrer" onerror="this.style.visibility='hidden'"/>`
+        : "";
       tr.innerHTML = `
         <td class="col-check"><input type="checkbox" data-idx="${i}" ${already ? "" : "checked"} /></td>
+        <td class="col-cover">${coverImg}</td>
         <td class="col-pos">${i + 1}</td>
         <td>${_escape(t.artists)}</td>
         <td>${_escape(t.name)}</td>
@@ -1752,8 +1769,12 @@ const YoutubeUI = (() => {
       const tr = document.createElement("tr");
       tr.dataset.idx = String(i);
       if (already) tr.classList.add("already-downloaded");
+      const coverImg = t.image_url
+        ? `<img src="${_escape(t.image_url)}" alt="" loading="lazy" referrerpolicy="no-referrer" onerror="this.style.visibility='hidden'"/>`
+        : "";
       tr.innerHTML = `
         <td class="col-check"><input type="checkbox" data-idx="${i}" ${already ? "" : "checked"} /></td>
+        <td class="col-cover">${coverImg}</td>
         <td class="col-pos">${i + 1}</td>
         <td>${_escape(t.title)}</td>
         <td>${_escape(t.channel)}</td>