Add gamepad support to the desktop build

Co-Authored-By: deepseek-v4.1-flash (opencode-go) via CodeCraft

tools/build_desktop.py

@@ -6,7 +6,8 @@
 
 - linux:   dist/DungeonCraft-<version>-x86_64.AppImage
            built with cargo-zigbuild against glibc 2.28, so it runs on distributions from about 2019 on;
-           needs on the build machine: zig, cargo-zigbuild, the ALSA headers (alsa-lib / libasound2-dev);
+           needs on the build machine: zig, cargo-zigbuild, the ALSA headers (alsa-lib / libasound2-dev)
+           and libudev (libudev-dev, for gamepads);
            appimagetool is downloaded once into ~/.cache/dungeoncraft/
 - windows: dist/DungeonCraft-<version>-windows-x64.exe  (one file, Windows 10 and newer)
            cross-compiled with cargo-zigbuild for x86_64-pc-windows-gnu