feat(glass): bake two-tier defaults (Adaptive chrome + media scope), add Starship/Matrix schemes

From UAT: glass reads strongest where there's content behind it, so split the
look into two tiers instead of one global compromise.
- index.css: global :root defaults are now the 'Adaptive' baseline (solid-enough
  dark glass for content-less chrome). New .glass-media scope class carries the
  UAT-tuned translucent values (blur 10 / surface 50% / card 60% / scrim 30% /
  edge 25%) for surfaces that float over real artwork.
- Apply .glass-media to the art-backed Plex detail views (PlexInfo page variant,
  PlexShowView, PlexSeasonView) and drop PlexInfo's hard-coded inline 55%
  overrides so those panels are scope-driven and consistent (hero no longer
  reads solid while the season strips are translucent).
- Two new colour schemes: Starship (deep-space blue, azure accent) and Matrix
  (near-black, phosphor green), each with dark+light tokens + swatches.
- Remove the thumbnail-mosaic backdrop (did nothing usefully; its home is the
  future 'videos behind glass' phase) — deletes MosaicBackdrop, mosaic vars/CSS,
  and the tuner's mosaic controls. GlassTuner presets resynced to the new
  baseline (Current / Media / Max-readable).

frontend/src/components/PlexBrowse.tsx

@@ -905,7 +905,7 @@ function PlexShowView({
   }
 
   return (
-    <div className="p-4 max-w-[1200px] mx-auto">
+    <div className="p-4 max-w-[1200px] mx-auto glass-media">
       <BackBtn onBack={onBack} label={t("plex.backToLibrary")} />
 
       {q.isLoading || !d || !show ? (
@@ -1103,7 +1103,7 @@ function PlexSeasonView({
   }
 
   return (
-    <div className="p-4 max-w-[1200px] mx-auto">
+    <div className="p-4 max-w-[1200px] mx-auto glass-media">
       <BackBtn onBack={onBack} label={t("plex.series.backToShow")} />
 
       {q.isLoading || !d || !se ? (