fix(ci): update snap test caps, install Flatseal in flatpak CI

- snap integration test asserted Clean=false, but snap now implements
  Cleaner — updated to assert True
- flatpak CI job now installs Flatseal before running integration tests
  so Version/List tests have a known installed package

.github/workflows/integration.yml

@@ -179,6 +179,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y flatpak
           sudo flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
+          sudo flatpak install -y flathub com.github.tchx84.Flatseal
       - name: Integration tests
         run: sudo -E go test -v -tags integration -count=1 -coverprofile=coverage-flatpak.out ./flatpak/
       - uses: actions/upload-artifact@v4