Fix accessibility audit findings

[BBalashi]

Resolves accessibility audit findings involving page landmarks, text spacing, status messages, language identification, responsive reflow, validation errors, focus visibility, and keyboard access to the confidence table.

Adds specific bilingual validation feedback, consolidates dynamic announcements into one polite atomic live region, improves narrow-screen wrapping and focus indicators, and updates documentation to preserve the accessible behavior.

Final checks included Git whitespace validation, static accessibility assertions, 26 validation cases, calculation regression testing, bilingual behavior checks, and live-region mutation testing.

[Garneauma]

From a technical standpoint, there is nothing that must be changed. It would have been better to have two pages, one in FR and one in EN instead of adding lang attributes everywhere, but it still works.