[Linux] TrayIcon: StatusNotifierItem Id is hard-coded to "nativeapi-tray"

Start in src/platform/linux/tray_icon_linux.cpp at the StatusNotifierItem Id property, then inspect how the default GApplication application ID and executable name are available. Trace the TrayIcon API and Flutter bindings to determine where an explicit id can be set before registration. Done means Linux icons have stable app-specific default ids, explicit ids are exposed, and other platforms follow the agreed no-op or mapping behavior.

On Linux, the tray icon is a StatusNotifierItem published by nativeapi, and its Id property is hard-coded (src/platform/linux/tray_icon_linux.cpp):

if (g_strcmp0(property_name, "Id") == 0)
  return g_variant_new_string("nativeapi-tray");

The SNI spec wants Id to be a name that is unique to the application and stable across sessions. Hosts use it to remember per-item settings (hidden/shown, order), and some show it to the user: niri shows it as the hover tooltip. Right now every nativeapi app reports the same nativeapi-tray, so hosts can't tell apps apart, and a user's choice for one app applies to all of them.

Proposal:

• Default the Id to something app-specific and stable, for example the application id (g_application_get_application_id() of the default GApplication, which is what Flutter's Linux runner sets from APPLICATION_ID), falling back to the executable name. Add a per-icon suffix only when an app has several tray icons.
• Let apps set it explicitly: TrayIcon::SetId(std::string) (or a constructor argument), set before the item is registered, since Id must not change afterwards. Expose it in the bindings (Flutter: TrayIcon.id).
• On macOS and Windows it can be a no-op, or map to NSStatusItem.autosaveName on macOS, which serves the same purpose (remembering position/visibility).

Background:

• leanflutter/tray_manager#97: incorrect ID when creating the app indicator on Linux (downstream: localsend/localsend#2902)
• leanflutter/tray_manager#100: expose the id for customization (niri shows it as the tooltip)