refactor: make Linux binary name lowercase

[schembriaiden]

Pull Request Description

I think in the next update this should be added as a breaking change at the top of the release to ensure that all package maintainers know about this.

This pull request standardizes the application binary name from Fladder (capitalized) to fladder (lowercase) across the entire codebase and related build/deployment files, to follow standards used by every Linux distro.

Application binary renaming:

• Changed the executable name variable in linux/CMakeLists.txt from Fladder to fladder to set the on-disk binary name to lowercase.
• Updated the program name in my_application.cc by calling g_set_prgname("fladder") instead of g_set_prgname("Fladder").

Build and packaging adjustments:

• Modified Flatpak build scripts and desktop entry files (flatpak/nl.jknaapen.fladder.yaml, flatpak/Fladder.desktop) to use fladder as the executable name, including the command field and file permissions.
• Updated the AppImage build configuration (AppImageBuilder.yml) to use fladder as the exec target.
• Changed the Linux artifact build and packaging workflow (.github/workflows/build.yml) to reference the lowercase binary name.

Screenshots / Recordings

Tested On

• Android
• Android TV
• iOS
• Linux
• Windows
• macOS
• Web

Checklist

• If a new package was added, did you ensure it works for all supported platforms? Is the package well maintained
• Check that any changes are related to the issue at hand.