style: modern dark UI refresh

- design-token palette (:root vars): deeper #0e0f13 base, indigo #7c8cff accent,
  layered surfaces, softer borders, shadow + radius scale
- tabs as floating rounded chips (active = elevated surface); refined dots
- key bar as rounded chips with gaps; Esc gets the accent tint
- toolbar icon buttons with rounded hover
- overlays (history/dashboard/qr/search/settings) get backdrop-blur + card +
  shadow; indigo primary buttons; consistent inputs with accent focus ring
- chrome uses a system UI font (terminal stays monospace)
- xterm dark theme + meta theme-color + manifest + app icon synced to the
  palette (indigo-gradient icon, accent cursor)
216 tests green; tsc + build clean.

public/index.html

@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover">
   <title>Web Terminal</title>
   <link rel="manifest" href="./manifest.webmanifest">
-  <meta name="theme-color" content="#1a1a1a">
+  <meta name="theme-color" content="#0e0f13">
   <link rel="apple-touch-icon" href="./icon.svg">
   <meta name="apple-mobile-web-app-capable" content="yes">
   <meta name="apple-mobile-web-app-status-bar-style" content="black">