Fix download modal unreachable after LiveCD removal

[Copilot]

After removing the #livecd/.modalbtn element from download.html, the JS click handler bound to .modalbtn became a no-op, and stripping href from #bootcd broke the link entirely (no navigation, no keyboard focus).

Changes

• static/js/download.js: Bind handler directly to #bootcd; retain its href as a no-JS fallback; call e.preventDefault() to intercept and show the modal when JS is active.

// Before (broken: .modalbtn no longer exists; href stripped)
var bootcd = $("#bootcd").attr("href");
$("#bootcd").removeAttr("href");
$(".modalbtn").click(function(e){ … });

// After
var bootcd = $("#bootcd").attr("href");
$("#bootcd").click(function(e){
  e.preventDefault();
  $("#bootcdModal").modal("toggle");
  $('.modal-footer a').attr("href", bootcd);
});

• content/download.html: Apply PR Reword the download.html page, removing mentions of deprecated LiveCD #138's content changes — removes the now-defunct LiveCD link and rewrites the download section copy to describe the unified Installation Medium.