refactor: 确立独立子工程架构 (backend, admin, mpuniapp) 并同步更新 AGENTS.md 规范

backend/.editorconfig

@@ -0,0 +1,11 @@
+# 🎨 editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
+insert_final_newline = true