fix(desktop): detect system dark mode on Linux

[tedin7]

Problem

On Linux, the desktop app ignores the system dark mode setting. The app always shows light theme even when the system is set to dark mode.

Root cause: WebKitGTK (used by Tauri on Linux) doesn't properly report prefers-color-scheme to CSS/JavaScript. It always returns light regardless of system settings.

Solution

1. Detect the system color scheme in Rust using gsettings (GNOME) with GTK theme fallback
2. Inject data-color-scheme attribute via initialization script before page load
3. Add CSS selector html[data-color-scheme="dark"] to apply dark theme

Changes

• packages/tauri/src-tauri/src/lib.rs - Add detect_linux_color_scheme() function
• packages/ui/src/styles/theme.css - Add dark theme selector for data-color-scheme attribute

Testing

Tested on Fedora 42 with GNOME dark mode enabled.

[rekram1-node]

A few things in flux atm, desktop guys requested I didn't merge PRs quite yet, they should be ready for contributions soon tho

[rekram1-node]

Ill have them review