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/icon.svg

@@ -1,6 +1,16 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512">
-  <rect width="512" height="512" rx="96" fill="#1a1a1a"/>
-  <g fill="none" stroke="#3fb950" stroke-width="34" stroke-linecap="round" stroke-linejoin="round">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#1a1c26"/>
+      <stop offset="1" stop-color="#0e0f13"/>
+    </linearGradient>
+    <linearGradient id="fg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#9d7cff"/>
+      <stop offset="1" stop-color="#7c8cff"/>
+    </linearGradient>
+  </defs>
+  <rect width="512" height="512" rx="112" fill="url(#bg)"/>
+  <g fill="none" stroke="url(#fg)" stroke-width="36" stroke-linecap="round" stroke-linejoin="round">
     <polyline points="150,196 226,256 150,316"/>
     <line x1="268" y1="320" x2="372" y2="320"/>
   </g>