blocks nest under their band, with a button per column

Adding a block means choosing which column it goes in, so a flat list cannot
express it. A one-column band shows one button and a two-column band shows two —
and a band with one column never has to answer a question it has no answer to.

The blocks are links, like everywhere else: pressing one opens its editor.

That is the last of the four actions the painted-on bars used to carry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/sections/PageBuilderPanel.tsx

@@ -920,6 +920,8 @@ export const PageBuilderPanel = () => {
           onAdd={() => handlersRef.current.add()}
           onDelete={(section) => handlersRef.current.remove(section.id)}
           onReorder={(ids) => void reorderRef.current(ids)}
+          onAddComponent={(section, column) => handlersRef.current.addComponent(section, column)}
+          onDeleteComponent={(block) => handlersRef.current.removeComponent(block)}
         />,
       );
       return;