fix(web): hide cast device row when no Cast/AirPlay target is detected

[raymondjacobson]

Problem

The player's Connect popup always listed a Google Cast devices (or AirPlay devices) row — even when there were no wireless targets on the network. With nothing to connect to, clicking it just opened an empty system picker, which is confusing.

Fix

Detect whether a wireless playback target is actually reachable, and only render the device row when one is (or while already casting, so the user can still disconnect). The "This web browser" option always remains.

• Chromium (Cast): RemotePlayback.watchAvailability() reports availability changes; cleaned up via cancelWatchAvailability().
• Safari (AirPlay): the webkitplaybacktargetavailabilitychanged event reports available / not-available.
• If a platform can't report availability (rejects with NotSupportedError, or the API is missing), we assume a device may be present so the cast flow is never hidden on capable browsers.

The cast icon itself is unchanged — only the in-popup device row is gated.

Changes

• useRemotePlayback.ts — new available state + availability-watching effect for both Chromium and Safari paths.
• CastButton.tsx — passes castAvailable down to the popup.
• ConnectPopup.tsx — conditionally renders the device row on castAvailable || isCasting.

Testing

• tsc --noEmit and eslint pass on the changed files.
• Live browser verification wasn't practical: the cast button only renders in the play bar with an active track, and availability depends on real Cast/AirPlay hardware on the network (absent in a headless preview, where the row correctly stays hidden — the new default).

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Preview URL: https://audius-web-preview-pr-14457.audius.workers.dev

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[raymondjacobson]

Closing — the availability gate here relies on the W3C Remote Playback API (audio.remote.watchAvailability()), which reports no devices when the audio element is routed through Web Audio via createMediaElementSource (as this app does in AudioPlayer.ts). That makes the gate hide the cast row even when Cast devices are present, which is worse than the original behavior. Accurate detection would require integrating the Google Cast sender SDK; deferring that for now.