core: load app defaults before applying base_settings / fix composer cleanup after install/update (#9965)

* fix(build): load app defaults before applying base_settings

App defaults were loaded after base_settings, causing saved values to be ignored.
Now loads var_* from app defaults file before calling base_settings.

* fix(cleanup): suppress composer root warning in cleanup_lxc

Composer's clear-cache command prompts for confirmation when run as root,
causing scripts to hang at 'Cleaning up' stage. Set COMPOSER_ALLOW_SUPERUSER=1
to suppress the interactive prompt.

Fixes #9952 (Heimdall Dashboard), also affects BentoPDF and other PHP apps.

* Fix COMPOSER_ALLOW_SUPERUSER export in cleanup_lxc

Exports COMPOSER_ALLOW_SUPERUSER before running composer clear-cache to ensure the environment variable is set correctly during cleanup.

misc/build.func

@@ -2148,8 +2148,8 @@ install_script() {
         header_info
         echo -e "${DEFAULT}${BOLD}${BL}Using App Defaults for ${APP} on node $PVEHOST_NAME${CL}"
         METHOD="appdefaults"
-        base_settings
         load_vars_file "$(get_app_defaults_path)"
+        base_settings
         echo_default
         defaults_target="$(get_app_defaults_path)"
         break