A11y: Missing toggled state information for ButtonTogglePanel

[rvveber]

https://github.com/suitenumerique/docs/blob/a4e3168682bdfa91f5a526d80cdeef02df73488c/src/frontend/apps/impress/src/features/header/components/ButtonTogglePanel.tsx#L7-L27

• Problem: The toggle button always announces “Open the header menu,” never updates based on isPanelOpen, lacks aria-expanded/aria-pressed, and doesn’t reference the controlled panel.
• Impact: Screen-reader users cannot tell whether the panel is open or closed or whether activating the control will open or close it.
• Acceptance criteria / fix ideas:
  • Update the label text depending on isPanelOpen (e.g., “Open…” vs “Close…”).
  • Add aria-expanded={isPanelOpen} (and optionally aria-controls pointing to the panel container) or make it a proper toggle with aria-pressed.