Brings in aarch64 host support (#152, #162, #167), natural scrolling (#163)
and the arm64 CI build job. Packaging, Pages and README keep the Nightcap
x86_64 side; upstream's aarch64 bundles are not published.
Rename the desktop entry, metainfo, icons, Flatpak manifest, and SDL app
identity so Nightcap installs side by side with upstream. Build a signed
Flatpak on v* tags, publish the OSTree repo and landing page to GitHub
Pages, and attach the bundle to the release. Release creation tolerates
the AppImage and Flatpak workflows racing on the same tag.
vii