Publish desktop releases to a self-hosted dnf repo

scripts/rpm-publish.sh builds the RPM, signs it and the repo metadata
with a dedicated GPG key, and adds it to a dnf repo in a given directory,
keeping the newest three versions for downgrades. The directory holds
nothing secret, so it can be served as is.

Both publish scripts now share the version scheme in
scripts/lib/release-version.sh: <major>.<minor>.<commit count>, plain
enough for RPM, MSI and semver alike. Android's versionCode stays the
commit count.

Fixes the publish scripts failing under set -u when the repo URL
variable isn't set, which is every run after the first. tauri.conf.json
now declares the license and homepage for the package metadata.

scripts/fdroid-publish.sh

@@ -108,10 +108,10 @@ fi
 # The versionCode is the commit count, so every commit on main can ship with no
 # manual bump and F-Droid sees each one as newer than the last.
 
-version_code="$(git rev-list --count HEAD)"
-base_version="$(node -p "require('./src-tauri/tauri.conf.json').version")"
-version_name="$base_version-$version_code-g$(git rev-parse --short HEAD)"
-if [ -n "$(git status --porcelain --untracked-files=no)" ]; then
+source scripts/lib/release-version.sh
+version_code="$release_count"
+version_name="$release_version"
+if $release_dirty; then
     if ! $allow_dirty; then
         err "the working tree has uncommitted changes; commit them or pass --allow-dirty"
         exit 1
@@ -121,7 +121,8 @@ fi
 
 # --- 4. The repo, the first time --------------------------------------------
 
-repo_url="${FDROID_REPO_URL%/}"
+repo_url="${FDROID_REPO_URL:-}"
+repo_url="${repo_url%/}"
 case "$repo_url" in
     "" | */repo) ;;
     *) err "FDROID_REPO_URL must end in /repo (fdroidserver requires it), e.g. $repo_url/repo"; exit 1 ;;