Docs site: unsupported locale in URL renders a blank page instead of a 404/fallback

## Problem

Navigating to the WebUI user manual docs site with an unsupported/invalid language code in the URL path results in a blank page, with no error message or fallback content.

## Suggested improvement

Add a proper fallback (e.g. a 404-style page, or a redirect to the default/nearest supported locale) when the requested locale is not supported, instead of rendering blank.

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Captured while working on branch: fix/fr-3269-webui-help-docs-url

JIRA Issue: FR-3280

Reproduce the WebUI user manual URL with an unsupported language code and trace the docs-site locale handling entry point. Done means the request shows a 404-style response or redirects to a supported default or nearest locale instead of rendering a blank page.

Written by the indexing model from the issue text.