feat(studio): play the scenario's audio while previewing (#182)

The studio had no audio output at all — no backend in its dependencies, no
stream anywhere, Present mode included. Judging a cut without sound is half
the job, so the transport now plays.

What it plays is the output of mix_audio_tracks, the same function the encoder
calls, so start/end, fades, per-track volume and volume keyframes are heard
exactly as they will be muxed. A preview that ignored the envelope would
misreport the edit.

The mix spans the whole scenario, silence included, which lets the playhead
follow the audio clock instead of a 1/fps timer that drifts against the sound
card over a long scenario. With no track — or no output device, as in CI — the
timer stays in charge and nothing changes.

cpal's stream is not Send, so it runs on its own thread behind a command
channel; position and state are published as atomics the UI reads without
touching it. Seeking publishes the new position on the caller's side so a
scrub cannot read back as the old one and snap the playhead backwards.
Play/pause, scrub and a mute button drive it; the mix is rebuilt whenever the
model reloads, so opening another file does not keep the previous sound.

Author: LeadcodeDev

Cargo.lock

@@ -22,6 +22,7 @@ serde_json = { version = "1", features = ["preserve_order"] }
 tokio = { version = "1", features = ["time"] }
 dioxus-icons = { version = "0.1.0", default-features = false }
 rfd = "0.17"
+rodio = { version = "0.22", default-features = false, features = ["playback"] }
 dirs = "6.0.0"
 palette = { version = "0.7.6", default-features = false }
 schemars = "0.8"