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/terminal-session.ts

@@ -107,7 +107,7 @@ export class TerminalSession {
     this.term = new Terminal({
       scrollback: 5000,
       fontFamily: 'Menlo, Consolas, monospace',
-      theme: { background: '#1a1a1a', foreground: '#e8e8e8', cursor: '#e8e8e8' },
+      theme: { background: '#0e0f13', foreground: '#e7e8ec', cursor: '#7c8cff' },
     })
     this.fitAddon = new FitAddon()
     this.term.loadAddon(this.fitAddon)