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/lib/theme.ts

@@ -1,13 +1,15 @@
 import { LS, readAccountMerged, writeAccount } from "./storage";
 
 type Mode = "dark" | "light";
-export type Scheme = "midnight" | "forest" | "slate" | "youtube";
+export type Scheme = "midnight" | "forest" | "slate" | "youtube" | "starship" | "matrix";
 
 export const SCHEMES: { id: Scheme; name: string; swatch: string }[] = [
   { id: "midnight", name: "Midnight", swatch: "#6d8cff" },
   { id: "forest", name: "Forest", swatch: "#2dd4bf" },
   { id: "slate", name: "Slate", swatch: "#e8913c" },
   { id: "youtube", name: "YouTube", swatch: "#ff3b46" },
+  { id: "starship", name: "Starship", swatch: "#38b6ff" },
+  { id: "matrix", name: "Matrix", swatch: "#2fe36e" },
 ];
 
 export interface ThemePrefs {