fix: eliminate UI freeze during cloud preset sync on startup
Move synchronous HTTP calls off the main thread in
GUI_App::start_sync_user_preset():
- Call scan_orphaned_info_files() and process_delete_presets() in
the background sync thread instead of the UI thread, so their
HTTP DELETE calls don't block startup.
- Call reload_settings() directly from the background thread
instead of via CallAfter, so get_user_presets() (HTTP GET) and
load/save_user_presets (file I/O) run off the main thread.
- Guard update_side_preset_ui() and app_config->save() with
is_main_thread_active() / CallAfter so they're safe when called
from a worker thread.
- Simplifiy finishFn lambdas: the progress-dialog case only
destroys the dialog; the no-dialog case is a no-op.
2167378bbe