fix: make the dev panel toggle keyboard operable

[Fasping]

Closes #230.

The click handler that opens the panel lived on the <svg> inside the toggle button rather than on the button itself. Activating a button with Enter or Space dispatches a click on the button, which never reaches the svg, so the panel could only be opened with a pointer.

• moves the handler to the <Button>
• marks the logo as decorative (aria-hidden, focusable="false") and drops its aria-label, so the button keeps a single accessible name
• adds aria-label="Show dev panel" to the button
• moves focus to the button that replaces the one being unmounted, in both directions
• gives the close button an accessible name; it was announcing as "✕"
• adds a test covering keyboard activation and focus

No visual or API change.

[bluebill1049]

Nice, thanks.