refactor(plugin-terminals): rework UI with antfu-design system

Restyle the terminal panel after the antfu / vitejs-devtools (rolldown)
aesthetic:

- Adopt presetWind4 with semantic tokens (bg-base, bg-secondary,
  border-base, color-active, op-fade, op-mute) and a green primary accent;
  light and dark are designed together and driven by the `.dark` class.
- Replace ad-hoc inline color classes with the token system and reusable
  shortcuts (btn-action, btn-icon, tab-item, status dots, mode badges).
- Cleaner shell: brand + session tabs with status dots and hover-close, a
  "+" icon button, a presets dropdown, and a details toolbar with restart /
  kill icon actions. Phosphor duotone icons throughout.
- Add a global stylesheet (base shell, themed scrollbars, xterm host) and
  tie the xterm themes to the app surface (off-black #111, primary cursor).

Author: antfubot