Use portable flatpak command in .desktop launcher

[digiArchitect]

Problem

The Exec line in .local/share/applications/org.gimp.GIMP.desktop hardcodes two things that break the PhotoGIMP launcher on some systems (reported in #127, and the arch half also noted in #200):

1. /usr/bin/flatpak — not all distros put the flatpak binary there. On NixOS /usr/bin contains only env, so clicking the PhotoGIMP icon silently does nothing. Per the Desktop Entry spec, a bare executable name is resolved via $PATH, which is what most Flatpak-exported .desktop files use.
2. --arch=x86_64 — on aarch64 machines (e.g. Asahi Linux, ARM laptops) this forces a non-installed architecture and the launcher fails. Removing the flag lets flatpak pick the native arch.

Change

-Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=gimp --file-forwarding org.gimp.GIMP @@u %U @@
+Exec=flatpak run --branch=stable --command=gimp --file-forwarding org.gimp.GIMP @@u %U @@

One line, no other changes.

How to test

1. Install GIMP from Flathub: flatpak install flathub org.gimp.GIMP
2. Copy the patched .desktop file to ~/.local/share/applications/ and run update-desktop-database ~/.local/share/applications (or log out/in).
3. Click the PhotoGIMP icon in the app menu — GIMP should launch. You can also test from a terminal with gio launch ~/.local/share/applications/org.gimp.GIMP.desktop or gtk-launch org.gimp.GIMP.
4. Regression check on a mainstream distro (Ubuntu/Fedora, x86_64): launcher still opens GIMP and image files still open via "Open with".
5. If available, repeat step 3 on NixOS (flatpak not in /usr/bin) and on an aarch64 machine — both previously failed silently, both should now launch.
6. Optional: desktop-file-validate org.gimp.GIMP.desktop reports no new errors.

Fixes #127