Adds optional 'browser' field to url-action; runtime resolves it via
config.BROWSER_PATHS (subprocess.Popen with the chosen exe). When
omitted, falls back to webbrowser.open (system default). Constructor
GUI gains a browser dropdown on URL action and on multi-step URL.
Existing open_browser/youtube/google updated to use yandex by default
since user prefers Yandex Browser.
Single window (900x720, dark background) loading web/index.html with a
Python bridge exposing list_existing_commands, list_exes,
list_sound_names, preview_sound, slugify, validate_draft, save_command,
plus llm_suggest hook for M3. Sound preview spawned on a daemon thread
so the GUI does not block.
Uses ruamel.yaml so existing comments / formatting in commands.yaml
survive a save. Each write creates commands.yaml.bak first, then
atomically renames a .tmp file into place. Tests cover round-trip for
all action types plus comment-preservation on append.
