Migrate the REST router off go-chi/chi onto go-pkgz/routegroup (backed by the
stdlib http.ServeMux), removing the last use of go-chi from the backend:
- rest.go routes() builds the tree with routegroup (Mount/Group/Route/With) and
net/http method+path patterns instead of chi's Get/Post/Route/Mount helpers
- chi.URLParam(...) -> r.PathValue(...) in the admin, public and private handlers
- rest_public_test.go loadPictureCtrl test uses routegroup + http.ServeMux
- rest_test.go: add TestRest_FileServerStaticAssets (bare /web -> /web/ redirect,
cache headers, 404, directory-listing block) and update the path-traversal test
for ServeMux normalising a literal ".." (encoded traversal is still rejected
by the handler)
- drop go-chi/chi from go.mod, go.sum and vendor; update the CLAUDE.md reference