The `app` feature (App / run_app, AppConfig, SiteHooks): registers the Tauri plugins every app
uses (single-instance, deep-link and updater opt-in, window-state, opener, process, notification,
dialog), makes app data, loads profiles and the language, builds the window (restore, lost-geometry
reset, helper-close routing, native resize follow) and the transparent shell, and serves tabs_sync,
tab_navigate, tab_action, open_external_url, shell_focus, shell_raise, window_system_menu,
profiles_get/set, i18n_get/set (shell-only) and tab_shortcut, tab_link_drag (tab-only). The tab
builder is generic; the app fills in SiteHooks. `commands` lists them (manifest() for build.rs),
`testkit` checks capability files and caller guards, i18n::page_script is origin-guarded.
check:tauri now checks the app feature.