Issue #3595614: translate the demo webforms on the French preview

On the Tugboat French demo (/fr), the two example webforms rendered in English. Two distinct causes, both fixed here (demo-only, in .tugboat/setup.php):

1. Form title + element labels are config, not interface strings, so the bundled/imported .po does not reach them. Fix: a French language config-translation override (title + elements) per demo webform; webform reads it via the config override language in WebformTranslationManager::getElements().

2. Webform framework strings (Submit button, "required", file-upload widget, validation) stayed English because the contrib .po loop resolves each URL from locale update-status release data the Tugboat image lacks for contrib -- so webform was skipped despite its .po being published (webform is a release candidate, but webform-6.3.0-rc1.fr.po exists on l.d.o). Fix: import webform's .po explicitly, building the l.d.o URL from the installed version.

Verifies on the Tugboat preview (the demo webforms are not built on local dev).

⚠️ The third commit (TEMPORARY) opens the base-only French gate to MR previews so this can be reviewed in the MR's preview. Revert it before merge.