fix(desktop): Restore Linux launcher icons and matching#3

What Changed

Linux launchers now advertise StartupWMClass=com.t3tools.T3Code, the window class and Wayland app id the app actually reports, in the electron-builder desktop entry and both AUR launchers. Before, they advertised t3code.

This is the same change as pingdotgg/t3code#10895 linux-wayland-desktop-icon. The icon discovery work this layer used to carry has since landed upstream in pingdotgg/t3code#8673 linux-mime-handler-cache.

Why

The running window's class comes from setDesktopName("com.t3tools.T3Code.desktop"), so with the old StartupWMClass GNOME, Cinnamon and similar shells matched the window to the hidden URL-handler entry instead of the launcher, and showed a second dock icon that cannot be pinned.

UI Changes

The running window now groups under the installed launcher's dock icon instead of a separate entry. No screenshots are attached.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes