feat(app): desktop notifications on Windows

Toasts through WinRT, under the app id io.narl.proton-stream. The id is
registered for the current user under HKCU\Software\Classes\AppUserModelId
with a display name and icon on the first notification of a session, which
is what lets an app with no package or installer-made shortcut show one. If
that write fails, the toast goes out under PowerShell's id rather than not
at all.

Cargo.toml

@@ -98,6 +98,8 @@ souvlaki = { version = "0.8", default-features = false, features = ["use_dbus"]
 # hold it for.
 dbus = "0.9"
 windows-sys = "0.61"
+# Toasts on Windows. Linux notifications go over the `dbus` above.
+tauri-winrt-notification = "0.8"
 rpassword = "7"
 libloading = "0.8"
 lru = "0.12"