Set the chart's placeholder version to 0.18.0

Cosmetic, and done anyway for the same reason as ee22eb0, 7b9a337 and
828cf87 before it: .gitea/workflows/release.yaml passes --version and
--app-version to `helm package` from the git tag, so neither line decides
anything about what is published. Being read is all they do, and a tree
heading for v0.18.0 that says 0.17.0 tells its reader something false.

appVersion keeps the v, per APPVERSION_PREFIX in .release.conf, and
image.tag in values.yaml stays "latest" -- that one is what a local
`helm install ./charts/terdut-server` actually pulls.

Claude-Session: https://claude.ai/code/session_01RHPj4ggeFdEjKKfm4SHbD7

charts/terdut-server/Chart.yaml

@@ -15,5 +15,5 @@ type: application
 # appVersion and image.tag in values.yaml no longer agree, and that is not an oversight:
 # image.tag stays "latest", which is what a local install actually pulls. appVersion is
 # metadata and drives nothing.
-version: 0.17.0
+version: 0.18.0
-appVersion: "v0.17.0"
+appVersion: "v0.18.0"