Sync sidebar/queue/player bar color transitions with the main background

Sidebar, Queue, PlayerBar, TabsBar, and the hero/spotlight surfaces all
had an extra CSS `transition: background 0.5s ease-out` (theme-transition/
glass-bar) layered on top of backgrounds that are already animated
frame-by-frame over 600ms by the hue-interpolation loop in +layout.svelte.
That extra transition made them visibly lag behind and take a different
amount of time than the main app-backdrop, which has no such transition
and therefore tracks the JS animation exactly. Drop the redundant
transition so every hue-driven surface updates in lockstep.

Also fixes a dangling --color-btn-hover: var(--mf-btn-bg-hover) that
referenced an undefined custom property (theme.css never defined
--mf-btn-bg-hover after an earlier partial commit), which silently
broke the secondary button's hover background.

web-player/src/routes/layout.css

@@ -121,14 +121,10 @@
 	@apply text-xs tabular-nums;
 }
 
-@utility theme-transition {
-	transition: background 0.5s ease-out;
-}
 @utility glass-bar {
 	background-image: var(--mf-bar-bg);
 	border-top: 1px solid var(--mf-hairline);
 	backdrop-filter: blur(24px);
-	transition: background 0.5s ease-out;
 }
 @utility grid-cards {
 	display: grid;