Fix #840: Update Flatpak manifest to GNOME 48 and repair binary verification

[SpvceR3ii]

This PR fixes the following issues with the Flatpak build:

• Updated runtime-version and sdk from 47 (EOL) to 48.

• Updated the binary sha256 to 2f424d29b8ab9d949534d844cd2a70ab926cb5fa52192a3913cdf39b1a0eb975 to match the current latest release.

• Added dest-filename: eigenwallet.deb to ensure the ar command can locate the source file during the build process.

Verified successful build and installation using flatpak-builder.

Closes #840

[binarybaron]

The SHA cannot be hardcoded. It should (and I believe is) being dynamically changed by the CI script.

[SpvceR3ii]

The reason I updated it manually was that the current manifest in the repo is pointing to a version where the SHA no longer matches the remote source, causing the build to fail locally and likely in any environment not using the internal CI hooks. If the CI script is intended to handle the SHA updates, it appears to be out of sync with the latest debian-x86_64 binary on the CDN.

[SpvceR3ii]

Posted another commit to re-integrate the only-arches flag. The SHA that's located in flatpak/org.eigenwallet.app.json will remain updated to the new hash as presented in the recent commits I have made.