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/state.svelte.js

@@ -3570,7 +3570,7 @@ export function initEvents() {
         resumeMain: () => {
             // Close crossfade: come back from the preview at volume 0 and
             // ramp to the user's level while the preview rings down.
-            api.playerFadeResume(PREVIEW_CROSSFADE_MS, player.muted ? 0 : player.volume);
+            api.playerFadeResume(PREVIEW_CROSSFADE_MS);
         },
     });