iwd: extract shared state/security label helpers

Both popup.c and gadget.c carried near-identical _state_label/_sec_label
helpers, with the gadget version using bare ints instead of the
Iwd_Security enum. Move to iwd/iwd_labels.{c,h} and use the enum
consistently.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/meson.build

@@ -10,6 +10,7 @@ e_iwd_sources = [
   'iwd/iwd_manager.c',
   'iwd/iwd_device.c',
   'iwd/iwd_network.c',
+  'iwd/iwd_labels.c',
   'ui/wifi_list.c',
   'ui/wifi_auth.c',
   'ui/wifi_hidden.c',