Add a reusable directory-picker modal backed by a new generic
POST /api/lm/browse-directory endpoint (browse logic extracted from the
recipe batch-import handler into py/utils/directory_browser.py) and wire
a browse button plus advisory validate-path feedback (POST
/api/lm/validate-path) into the settings path inputs: recipes path,
example images path/local root, and the extra-folder/model-path rows.
The browse button insets into the right edge of static inputs so narrow
settings rows keep their single-control layout.
Translations for the new settings.directoryPicker and
settings.pathValidation keys are filled in for all 9 locales.
