feat(12.2-03): add hasMany relation contracts, relation forms and child create

- RelationContract gains Kind (empty is belongsToMany) and ForeignKey, with kind-aware boot checks
- manage.form, view.form and pivot.form compile against the related or pivot model; $/ paths resolve inside the plugin
- view toolbarButtons accept create|update|delete|link|unlink, each the capability of its routes
- POST .../relations/{name}/records creates a child through the manage form; the server sets the hasMany key
- relation schema carries kind, deferrable and the localized forms; 17 new relation message keys in en and pl

modules/phrasebook/backend/lang/en/lang.yaml

@@ -136,3 +136,22 @@ messages:
       one: "Unlinked :count record"
       other: "Unlinked :count records"
     empty: No linked records.
+    create: New record
+    create_title: New record
+    update_title: Edit record
+    preview_title: Record preview
+    created: Record created
+    updated: Record saved
+    delete_selected: Delete selected
+    delete_confirm: "Delete the selected (:count)? This cannot be undone."
+    delete_one_confirm: Delete this record? This cannot be undone.
+    deleted:
+      one: "Deleted :count record"
+      other: "Deleted :count records"
+    pivot_title: Link details
+    pivot_saved: Link details saved
+    edit_pivot: "Edit link details: :name"
+    create_submit: Create record
+    update_submit: Save record
+    pivot_submit: Save link details
+    link_submit: Add link