API keys set via /ai set token were only stored in memory and lost
on client restart. prefs_ai_set_token() and prefs_ai_remove_token()
mutated the in-memory prefs but never called _save_prefs(), unlike
other preference setters in the same file.
Both functions now call _save_prefs() after mutating the keyfile,
ensuring API keys persist across client restarts as advertised.