fix(packaging): point the packaged systemd unit at /usr/bin/punktfunk-host

scripts/punktfunk-host.service is dev-oriented — its ExecStart references the
source tree (%h/punktfunk/target/release/punktfunk-host). When the deb/rpm ship
it to /usr/lib/systemd/user, a fresh install with no hand-rolled unit would try
to run a binary that isn't there. Rewrite the ExecStart to the installed
/usr/bin/punktfunk-host during packaging (sed in build-deb.sh + the spec); the
source unit stays as-is for from-source dev. Hosts with a custom ~/.config unit
(which shadows the packaged one) are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/rpm/punktfunk.spec

@@ -136,6 +136,9 @@ install -Dm0644 scripts/99-punktfunk-net.conf %{buildroot}%{_prefix}/lib/sysctl.
 
 # systemd *user* unit (the host runs in the graphical session, not as root).
 install -Dm0644 scripts/punktfunk-host.service %{buildroot}%{_userunitdir}/punktfunk-host.service
+# The source unit's ExecStart points at the dev source tree; a packaged install has the binary at
+# %{_bindir}. Rewrite it so a fresh install (no hand-rolled unit) starts the installed binary.
+sed -i 's#%h/punktfunk/target/release/punktfunk-host#%{_bindir}/punktfunk-host#' %{buildroot}%{_userunitdir}/punktfunk-host.service
 
 # --- client subpackage ---
 install -Dm0755 target/release/punktfunk-client %{buildroot}%{_bindir}/punktfunk-client