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/lib/components/layout/Sidebar.svelte

@@ -31,7 +31,7 @@
 </script>
 
 <aside
-	class="hidden shrink-0 flex-col border-r border-hairline bg-bg p-5 theme-transition lg:flex"
+	class="hidden shrink-0 flex-col border-r border-hairline bg-bg p-5 lg:flex"
 	style="width:var(--mf-sidebar-w)"
 >
 	<nav class="flex flex-col gap-1">