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/manifest.webmanifest

@@ -6,8 +6,8 @@
   "scope": "./",
   "display": "standalone",
   "orientation": "any",
-  "background_color": "#1a1a1a",
-  "theme_color": "#1a1a1a",
+  "background_color": "#0e0f13",
+  "theme_color": "#0e0f13",
   "icons": [
     {
       "src": "./icon.svg",