Bricks Builder MCP v1.6.4

Add append_elements and append support

Introduce Bricks_Data::append_elements to append new elements to an existing area, resolving ID collisions by remapping IDs, updating parent/child references, and supporting insertion after a specific element's root tree (collect_tree_ids helper). Update pages, posts, and templates tools to accept append and insert_after parameters, update input schemas/descriptions, and return an appended flag and contextual messages. Improve admin.js memory loading with clearer error messages and robust AJAX fail handling, and always trigger memory load when the memory tab is clicked. Also update the plugin header and BMCP_VERSION (1.7.0 -> 1.6.4).