Add dark mode support for the Windows desktop app

[lost9999]

Background

PromptX on Windows desktop is currently hard to use for long sessions in low-light environments because the UI is light-only. Please add dark mode support for the Windows desktop app.

Expected behavior

• The Windows desktop app supports a dark theme.
• The app can follow the Windows system theme automatically.
• The app also offers an in-app theme setting: System / Light / Dark.
• Dark mode covers core pages, dialogs, inputs, buttons, sidebars, scrollbars, and status messages with readable contrast.

Suggested acceptance criteria

• On Windows 11 with the system dark theme enabled, the app starts in dark mode by default.
• Changing the theme in the app takes effect immediately, or the app clearly says a restart is required.
• Core desktop pages do not show large light-only areas, flashes, or missing theme styles in dark mode.

Why this matters

This improves day-to-day usability for Windows users, especially during night work, long MCP/Agent debugging sessions, OLED displays, and low-brightness environments.