Migrate desktop UI to Tailwind CSS 4 and Ant Design

[Mieluoxxx]

Summary

Modernize the desktop frontend styling and component layer by upgrading to Tailwind CSS 4 and adopting Ant Design for interactive UI controls.

Scope

• Upgrade Tailwind and PostCSS integration to the CSS-first Tailwind 4 setup.
• Add an Ant Design theme bridge for the existing semantic light/dark palette.
• Replace native buttons, inputs, selects, switches, dialogs, popovers, confirmations, and feedback messages with Ant Design components.
• Keep Tailwind utilities for application layout and semantic surface styling.
• Reorganize Settings around user-facing categories: General, AI and agents, Knowledge processing, Connections and sync, and System.
• Preserve responsive behavior at the desktop minimum window size.

Acceptance criteria

• No application-authored native button, input, textarea, or select controls remain.
• Browser confirm() and alert() calls are replaced with themed Ant Design feedback.
• Light and dark themes remain aligned with the existing semantic palette.
• Settings controls are consistently sized and right-aligned.
• npm run lint and npm run build pass.
• Chat, Library, Search, Settings, Help, and About render without clipping at 900x600.