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/lib/release-version.sh

@@ -0,0 +1,18 @@
+# release-version.sh — the version every published build carries.
+#
+# SOURCE this from the repo root. It sets:
+#
+#   release_count    commits on HEAD; only ever goes up on main
+#   release_version  <major>.<minor>.<release_count>, e.g. 0.1.47
+#   release_dirty    true when tracked files have uncommitted changes
+#
+# Major and minor come from tauri.conf.json; the patch is the commit count, so
+# any commit can ship without a manual bump, and the number is plain enough for
+# every package format (Android versionCode, RPM, MSI, semver).
+
+release_count="$(git rev-list --count HEAD)"
+release_version="$(node -p "require('./src-tauri/tauri.conf.json').version.split('.').slice(0, 2).join('.')").$release_count"
+release_dirty=false
+if [ -n "$(git status --porcelain --untracked-files=no)" ]; then
+    release_dirty=true
+fi