Fase B2: give Contents root semantics alongside Library

Adds a shared common-config/contents-page-name constant and
ldb/contents?/get-contents-page, mirroring the existing Library
helpers. Contents already qualifies as an internal page, so the
existing namespace-title split mechanism (e.g. "Contents/Proyectos")
already parents pages under it via :block/parent, coexisting with
Library rather than replacing it as the default root.

deps/common/src/logseq/common/config.cljs

@@ -59,6 +59,7 @@
 (defonce favorites-page-name "$$$favorites")
 (defonce views-page-name "$$$views")
 (defonce library-page-name "Library")
+(defonce contents-page-name "Contents")
 (defonce quick-add-page-name "Quick add")
 (defonce recycle-page-name "Recycle")