Mobile: live external audio input (USB) source selector

Expose the engine's live-input mode on the remote: an "Sorgente audio"
selector in the Audio tab lists the input devices the output reports and
switches between File/Playlist and a live USB capture (useInput); picking
File again stops the input. Grant the kiosk getUserMedia access with
--use-fake-ui-for-media-stream so device enumeration/labels work without a
prompt. Make reportDevices resilient (per-probe timeout, always report even
an empty list) so the UI gets feedback when no input is connected yet.

The Pi 5 has no audio input — a USB (line) sound card is required; visuals
react to the source while the audio stays on the mixer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/css/mobile.css

@@ -34,6 +34,7 @@ button, select, input, textarea {
 button { cursor: pointer; }
 button:active { filter: brightness(1.3); }
 button.wide { width: 100%; margin-top: 10px; font-size: 15px; }
+#audio-src { width: 100%; }
 input[type=range] { -webkit-appearance: none; appearance: none; background: transparent; padding: 0; height: 34px; border: none; }
 input[type=range]::-webkit-slider-runnable-track { height: 6px; border-radius: 4px; background: rgba(255,255,255,.14); }
 input[type=range]::-webkit-slider-thumb { -webkit-appearance: none; width: 24px; height: 24px; border-radius: 50%; background: var(--accent); margin-top: -9px; box-shadow: 0 1px 4px rgba(0,0,0,.5); }