fix(glass): give light theme its own translucent tier + unify the Plex filter rail

Light theme has no background image, so the adaptive :root glass (94%) rendered
as flat snow-white islands, while the Plex filter rail (still on bg-surface/40)
showed the ambient tint — an inconsistency. Fix centrally, not per-component:
- index.css: one html[data-theme="light"] token override (surface 62 / card 74 /
  menu 86) so every .glass* surface is translucent in light and picks up the
  ambient tint, like it refracts the backdrop in dark.
- PlexSidebar: bg-surface/40 -> glass, so all filter rails bind to the same token.
One central lever now controls the light-theme glass translucency for the whole app.

frontend/src/components/PlexSidebar.tsx

@@ -120,7 +120,7 @@ export default function PlexSidebar({
   }
 
   return (
-    <aside className="hidden md:block w-64 shrink-0 border-r border-border bg-surface/40 overflow-y-auto p-4 space-y-4">
+    <aside className="hidden md:block w-64 shrink-0 glass overflow-y-auto p-4 space-y-4">
       {/* Collapse control + active-filter count — mirrors the feed Sidebar so the Plex filter panel
           can be collapsed from the Plex page too (the collapsed rail + shared state already existed). */}
       <div className="flex items-center gap-1.5 min-w-0">