conversions web + put table definition

web/src/pages/admin/table_definition/state.rs

@@ -384,6 +384,12 @@ impl TableDefinitionPageState {
         self.table_is_writable() && self.detail.as_ref().is_some_and(|detail| !detail.has_data)
     }
 
+    /// How many columns the table already has, for the unified "Columns"
+    /// count the append panel shows alongside the create builder's own.
+    pub(crate) fn existing_column_count(&self) -> usize {
+        self.detail.as_ref().map_or(0, |detail| detail.columns.len())
+    }
+
     pub(crate) fn column_is_selected_for_removal(&self, column_id: &i64) -> bool {
         self.remove_column_ids.contains(column_id)
     }