Function load_start_page_from_disk 

pub fn load_start_page_from_disk(config_watcher: &ConfigState)

Load browser-start-page.json from disk into the ConfigState. Called once at startup, mirroring config_watcher_fs::load_settings_from_disk. An unresolvable shared_dir or a missing file both leave the config’s browser_start_page at its default (None) rather than failing boot — a start page is a convenience, never a startup precondition.