v1.7.3: nuova icona app + landing chiarisce limiti senza chiavi Spotify

Icona: rigenerati icon.icns (macOS multi-size) e icon.ico (Windows
multi-size 16-256) dalla sorgente assets/icon_1024.png.

Landing: nuovo box "Per sfruttare al massimo MusicTools serve un
account Spotify" + FAQ esplicita su cosa funziona senza setup
e dove servono le chiavi.

landing/css/style.css

@@ -601,6 +601,69 @@ a { color: inherit; text-decoration: none; }
   text-align: center;
 }
 
+/* Box informativo sulle chiavi API Spotify */
+.spotify-note {
+  max-width: 900px;
+  margin: 48px auto 0;
+  padding: 28px 32px;
+  background: var(--bg-card);
+  border: 1px solid var(--border);
+  border-radius: 18px;
+}
+.spotify-note h3 {
+  margin: 0 0 10px;
+  font-size: 19px;
+  font-weight: 800;
+}
+.spotify-note > p {
+  margin: 0 0 18px;
+  color: var(--text-muted);
+  font-size: 15px;
+  line-height: 1.55;
+}
+.spotify-note-grid {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 18px;
+  margin: 18px 0 14px;
+}
+@media (max-width: 640px) {
+  .spotify-note-grid { grid-template-columns: 1fr; }
+}
+.sn-col {
+  padding: 18px 20px;
+  border-radius: 12px;
+  border: 1px solid var(--border);
+}
+.sn-with {
+  background: rgba(29, 185, 84, 0.08);
+  border-color: rgba(29, 185, 84, 0.3);
+}
+.sn-without {
+  background: rgba(120, 120, 120, 0.06);
+}
+.sn-title {
+  font-weight: 800;
+  font-size: 14.5px;
+  margin-bottom: 8px;
+  color: var(--text);
+}
+.sn-col ul {
+  margin: 0;
+  padding-left: 18px;
+  color: var(--text-muted);
+  font-size: 14px;
+  line-height: 1.55;
+}
+.sn-col li { margin-bottom: 4px; }
+.sn-col strong { color: var(--text); }
+.spotify-note-fine {
+  margin-top: 12px;
+  font-size: 12.5px;
+  color: var(--text-dim);
+  text-align: center;
+}
+
 /* ============================================================
    FAQ
    ============================================================ */