Bundle fallback ringback and busy tone audio files

Most desktop Linux sound themes lack telephony events like
phone-outgoing-calling and phone-outgoing-busy, causing gsound
playback to fail silently. Bundle OGG Vorbis fallback files and
pass GSOUND_ATTR_MEDIA_FILENAME alongside GSOUND_ATTR_EVENT_ID
so libcanberra falls back to the bundled audio when the theme
event is not found.

data/meson.build

@@ -110,3 +110,10 @@ if compile_schemas.found()
     args: ['--strict', '--dry-run', meson.current_source_dir()],
   )
 endif
+
+# Sounds (fallback for systems without telephony events in sound theme)
+install_data(
+  'sounds/phone-outgoing-calling.oga',
+  'sounds/phone-outgoing-busy.oga',
+  install_dir: join_paths(datadir, calls_name, 'sounds'),
+)