fix: macOS "Where is use_default?" dialog when sending first notification

[Crempa]

Problem

When an agent finishes and Arbor sends its first desktop notification, macOS shows a "Choose Application" dialog asking "Where is use_default?" followed by a notification permission prompt. The user has to dismiss the dialog manually.

Root cause

The notify-rust crate depends on mac-notification-sys, which calls ensure_application_set() on the first notification. This function looks up the bundle identifier for a hardcoded app name "use_default":

// mac-notification-sys/src/lib.rs:110
let bundle = get_bundle_identifier_or_default("use_default");

Since no application named use_default exists, macOS presents an app chooser dialog. If dismissed, the library falls back to com.apple.Finder.

Suggested fix

Call mac_notification_sys::set_application("so.pen.arbor") early in the app lifecycle (e.g. in main() or app_init), before any notification is sent. This pre-sets the bundle identifier and skips the "use_default" fallback entirely.

Alternatively, notify-rust exposes set_application() directly — no need to depend on mac-notification-sys as a direct dependency.