fix: preview crossfade no longer strands the main volume (silent playback)

Clicking a track could play silently: the store showed status=playing with
the position advancing and no error, but the sink was at volume 0. The
open/close crossfade ramped the main player's USER VOLUME as its fade
channel and left it at 0 on an ack timeout (or when a track change
suppressed the close resume). User volume persists across tracks and a new
Play doesn't reset it, so every later click played at 0 while the slider
still read 0.7.

Give the main player a separate fade_gain channel (Cmd::SetFadeGain, sink =
volume * norm * fade_gain), reset to 1 by every Play/Stop. The open/close
fades ramp fade_gain instead of set_volume (their volume arg is gone, JS
callers too), so the user volume is never touched -- store and backend can't
desync -- and a clicked track always resets fade_gain to 1, so it can never
be stranded silent. fade_resume lands exactly at 1. Player::fade_gain()
mirrors it into a report atomic; debug overlay shows main fade + this proved
the fix live (open 1->0, close 0->1, new track 1.0, volume steady at 0.7).

39 app + 12 player + 85 JS tests pass.

src/lib/api.js

@@ -138,10 +138,10 @@ export const streamWaveform = (username, path, size, buckets) =>
 export const previewResolve = (trackId) => invoke("preview_resolve", { trackId });
 export const previewLoad = (leaseId, startPaused, crossfadeMs = 0, startAtSecs = null) =>
     invoke("preview_load", { leaseId, startPaused, crossfadeMs, startAtSecs });
-export const playerFadePauseAcked = (fadeMs, volume) =>
-    invoke("player_fade_pause_acked", { fadeMs, volume });
-export const playerFadeResume = (fadeMs, volume) =>
-    invoke("player_fade_resume", { fadeMs, volume });
+export const playerFadePauseAcked = (fadeMs) =>
+    invoke("player_fade_pause_acked", { fadeMs });
+export const playerFadeResume = (fadeMs) =>
+    invoke("player_fade_resume", { fadeMs });
 export const debugStats = () => invoke("debug_stats");
 export const previewControl = (action, value = null) =>
     invoke("preview_control", { action, value });