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/+page.svelte

@@ -135,7 +135,7 @@
 
 <Page>
 	<div
-		class="relative overflow-hidden rounded-panel-lg hero-surface px-6 py-8 theme-transition sm:px-9 sm:py-8.5"
+		class="relative overflow-hidden rounded-panel-lg hero-surface px-6 py-8 sm:px-9 sm:py-8.5"
 	>
 		<div
 			class="relative flex flex-col items-start gap-7 lg:flex-row lg:flex-wrap lg:items-end lg:justify-between"
@@ -211,7 +211,7 @@
 		{#if spotlight}
 			<section>
 				<div
-					class="grid grid-cols-1 overflow-hidden rounded-panel-lg spotlight-surface theme-transition lg:grid-cols-2"
+					class="grid grid-cols-1 overflow-hidden rounded-panel-lg spotlight-surface lg:grid-cols-2"
 				>
 					<div class="flex flex-col justify-center p-8 sm:p-9">
 						<div