feat(design-system): adopt cyan as data color, add Instrument Serif, introduce label roles (#674)

- Repoint --chart-1..3 to brand/warning/destructive so cyan leads as the
  primary data color, amber marks peaks, rose marks errors. --chart-4/5
  fall back to neutral greyscale tones tuned per theme.
- Add --data-line-width (1.25px) and --data-peak tokens so sparklines
  across dashboard, stack table, and readiness views share one source
  of truth instead of re-deriving per component.
- Introduce five semantic label role aliases (env, role, owner, risk,
  custom) mapped to existing hues. Raw ten-hue palette stays in place
  for existing consumers; new labels should consume roles.
- Replace Georgia serif with Instrument Serif and add --font-display
  alias in both :root and .dark; expose font-display via @theme inline
  so Tailwind utility classes can use it. Load the family from Google
  Fonts alongside Geist + Geist Mono.

frontend/index.html

@@ -8,7 +8,7 @@
   <title>Sencho</title>
   <link rel="preconnect" href="https://fonts.googleapis.com" />
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
-  <link href="https://fonts.googleapis.com/css2?family=Geist:wght@100..900&family=Geist+Mono:wght@100..900&display=swap" rel="stylesheet" />
+  <link href="https://fonts.googleapis.com/css2?family=Geist:wght@100..900&family=Geist+Mono:wght@100..900&family=Instrument+Serif:ital,wght@0,400;1,400&display=swap" rel="stylesheet" />
   <script src="/theme-init.js"></script>
 </head>