feat: extract cms from mp/cms — initial libreshop/cms

Source moved verbatim from mp/cms/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/cms on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

.dockerignore

@@ -0,0 +1,8 @@
+.tmp
+.cache
+.git
+build
+node_modules
+.env
+public/uploads/*
+package-lock.json