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/db/src/logseq/db.cljs

@@ -577,6 +577,11 @@
   [db]
   (get-built-in-page db common-config/library-page-name))
 
+(def contents? db-db/contents?)
+(defn get-contents-page
+  [db]
+  (get-built-in-page db common-config/contents-page-name))
+
 (defn get-case-page
   "Case sensitive version of get-page. For use with DB graphs"
   [db page-name-or-uuid]

deps/db/src/logseq/db/common/initial_data.cljs

@@ -431,7 +431,7 @@
         structured-datoms (get-structured-datoms db)
         user-datoms (get-all-user-datoms db)
         list-style-datoms (get-list-style-values db)
-        pages-datoms (let [contents-id (get-first-page-by-title db "Contents")
+        pages-datoms (let [contents-id (get-first-page-by-title db common-config/contents-page-name)
                            capture-page-id (:db/id (db-db/get-built-in-page db common-config/quick-add-page-name))
                            views-id (get-first-page-by-title db common-config/views-page-name)
                            recycle-id (get-first-page-by-title db "Recycle")]

deps/db/src/logseq/db/frontend/db.cljs

@@ -139,3 +139,11 @@
   [page]
   (and (entity-util/built-in? page)
        (= common-config/library-page-name (:block/title page))))
+
+(defn contents?
+  "Whether `page` is the built-in Contents page, the other hierarchical
+   root pages can be explicitly organized under (coexists with Library,
+   which remains the default root for namespaced pages)."
+  [page]
+  (and (entity-util/built-in? page)
+       (= common-config/contents-page-name (:block/title page))))

deps/db/src/logseq/db/sqlite/create_graph.cljs

@@ -130,7 +130,7 @@
 (def built-in-pages-names
   #{common-config/library-page-name
     common-config/quick-add-page-name
-    "Contents"})
+    common-config/contents-page-name})
 
 (defn- validate-tx-for-duplicate-idents [tx]
   (when-let [conflicting-idents