Radio: real variety, honest volume control

[jay23606]

Reported: the radio felt like a small mix again, and the volume would not adjust.

• Cause of the small mix: after one search the radio queued the whole page (20 tracks) from that single genre and played it out. Now every song comes from a fresh genre and random page, one track each, never the artist who just played. Genre list grown from 36 to 59.
• Volume: on desktop the slider is verified to set the element volume. Phones such as iPhones ignore an audio element's volume, and the track host allows CORS only from localhost, so it cannot be routed through Web Audio. The slider is now hidden while a real song plays on such a device, and the toggle's tooltip says to use the device buttons, instead of a slider that silently does nothing. The synth fallback always keeps its slider.
• Verified in the running app: five songs, five genres, slider moves the real volume. 501 tests, lint clean.

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