Summary

This PR adds a full dark theme for the current gui (Vue 2 + Buefy) with a Material 3-based palette, plus a navbar theme toggle with persistence.

The implementation is isolated in a dedicated stylesheet and avoids changing the existing light theme design logic.

What’s included

• Added dark theme toggle in navbar (App.vue)
  • Uses existing MDI icons (weather-night / weather-sunny)
  • Label switches dynamically (Dark/Light)
  • Persists in localStorage (theme=dark|light)
• Added dark theme stylesheet:
  • gui/src/assets/scss/dark-theme.scss
• Added i18n keys for theme toggle labels in existing locales:
  • en, zh, ru, pt-br, fa-ir

Styling approach

• Material 3 dark color roles via CSS variables (--md-*)
• Dark styles applied under body.theme-dark
• Kept base/light styles intact; dark theme is override-only

Covered UI states/components

• Navbar/menu/dropdowns (desktop + mobile menu)
• Buttons (default, outlined, primary, warning/info/success/danger/delete, hover/focus/active)
• Tabs (is-toggle, is-toggle-rounded, is-boxed, including focus/hover/active)
• Tables (header/body, sorted column state, unselectable row behavior)
• Inputs/textarea/select (including placeholders and select arrow)
• Primary checkboxes/switches (checked/hover/focus + icon color)
• Modals/cards/messages/sidebar
• Scrollbars and dividers

Notes

• No backend logic changes.
• No new dependencies added.
• Frontend build succeeds locally with existing non-blocking bundle-size warnings.