feat(design): the scheme set, the colour contract and the token utilities (E28 S3a)

Four schemes (Midnight; Forest cyan, Slate fuchsia, Matrix lime), YouTube
and Starship retired with no data migration: normalizeTheme maps a stored
retired scheme to Midnight. A vitest contract measures every scheme x mode
(text 4.5:1, control edge 3:1, status text, the accent's 30-degree hue gap);
it caught light-mode danger at 4.43:1, now red-700. The state layers, the
strong edge, the touch target and the fades are index.css tokens and
utilities the lab now reads. Backdrops recoloured to the new accents, with
a grey overlay grain that actually breaks the banding (longest flat run
15-26px -> 4-8px). The glass tuner is retired.

frontend/src/i18n/locales/en/settings.json

@@ -33,8 +33,6 @@
     "performanceModeHint": "Turns off the translucent glass blur and soft shadows for snappier rendering on slower machines.",
     "showHints": "Show hints",
     "showHintsHint": "These little hover explanations. Turn them off once you know your way around.",
-    "showTuner": "Show the glass tuner",
-    "showTunerHint": "A floating panel for live-tweaking the frosted-glass look. A geek toy — off by default.",
     "startup": "Startup",
     "startOnHome": "Start on Home",
     "startOnHomeHint": "Every launch opens “What should I watch?”. Off = pick up on the module you left open.",
@@ -249,60 +247,5 @@
     "clear": "Clear",
     "empty": "No event matches the filter.",
     "emptyAll": "No events yet."
-  },
-  "tuner": {
-    "open": "Open the glass tuner",
-    "tab": "◐ GLASS",
-    "title": "◐ Glass Tuner",
-    "lab": "lab",
-    "collapse": "Collapse",
-    "treatment": "Dark treatment",
-    "tabs": {
-      "glass": "Glass",
-      "palette": "Palette"
-    },
-    "paletteHint": "Overrides the live scheme (inline on <html>). Reset clears these back to the scheme.",
-    "export": "Export → bake in",
-    "syncHint": "Load the values of the theme you're currently viewing (light or dark) into the sliders",
-    "sync": "⟳ Sync to current theme",
-    "copy": "Copy CSS",
-    "copied": "Copied ✓",
-    "reset": "Reset",
-    "slider": {
-      "blur": "Blur radius",
-      "saturate": "Saturation",
-      "darkBright": "Dark brightness",
-      "surfaceAlpha": "Panel opacity",
-      "cardAlpha": "Card opacity",
-      "menuAlpha": "Menu opacity",
-      "borderAlpha": "Border opacity",
-      "edge": "Edge-light rim",
-      "inset": "Top highlight",
-      "scrim": "Under-text scrim",
-      "ambient": "Ambient intensity"
-    },
-    "color": {
-      "bg": "Background",
-      "surface": "Surface",
-      "card": "Card",
-      "border": "Border",
-      "fg": "Text",
-      "muted": "Muted",
-      "accent": "Accent"
-    },
-    "preset": {
-      "current": {
-        "name": "Glass/image",
-        "hint": "The default (over image)"
-      },
-      "solid": {
-        "name": "Solid (flat)",
-        "hint": "As shown w/ image off"
-      },
-      "readable": {
-        "name": "Max readable",
-        "hint": "Solid + strong scrim"
-      }
-    }
   }
 }