Video tab: modalita 'Solo audio' (estrazione MP3)

Aggiunto toggle Video/Solo audio nella tab Video. In modalita audio
yt-dlp scarica solo bestaudio ed estrae in MP3 al bitrate impostato
in Impostazioni (default 320 kbps), con embed thumbnail come la tab
Scarica.

- core/downloader.py: download_video() accetta audio_only e
  audio_bitrate; quando True usa format 'bestaudio/best' e
  --extract-audio --audio-format mp3
- api/bridge.py: start_video_download passa audio_only nel worker;
  log mostra 'Solo audio MP3 320K' vs 'Video 1080p'
- webui: segmented control 'Video (MP4)' / 'Solo audio (MP3)' rosa,
  selettore qualita nascosto in modalita audio, hint aggiornato

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

webui/css/style.css

@@ -615,6 +615,36 @@ input[type="number"]::-webkit-inner-spin-button {
 .log .l-warn { color: #FDCB6E; }
 .log .l-muted { color: var(--text-3); }
 
+/* ===============================
+   SEGMENTED CONTROL
+   =============================== */
+.segmented {
+  display: inline-flex;
+  background: var(--bg-input);
+  padding: 4px;
+  border-radius: var(--r-pill);
+  gap: 2px;
+}
+
+.seg-btn {
+  border: none;
+  background: transparent;
+  color: var(--text-2);
+  padding: 8px 18px;
+  border-radius: var(--r-pill);
+  cursor: pointer;
+  font-weight: 700;
+  font-size: 13px;
+  transition: background-color 0.15s, color 0.15s;
+}
+
+.seg-btn:hover:not(.active) { color: var(--text); }
+
+.seg-btn.active {
+  background: var(--pink);
+  color: #fff;
+}
+
 /* ===============================
    HINT BOX
    =============================== */