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.

.prettierrc.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://json.schemastore.org/prettierrc",
+  "semi": true,
+  "tabWidth": 4,
+  "singleQuote": false,
+  "printWidth": 150,
+  "trailingComma": "none",
+  "useTabs": true
+}