feat(a11y): Give the mobile menu button an accessible name and aria state

Upstream did this in header-content.hbs (b35fcda, 86237c9), a partial this
repo replaced with its own header.hbs, so the change could not be ported and
is reapplied here. Ours targets the popup-menu container rather than
upstream's topbar-nav.

The button previously had no accessible name at all, and nothing exposed its
open/closed state; 05-mobile-navbar.js already maintains aria-expanded and now
reads aria-controls, falling back to data-target.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lfrancke

ui/src/partials/header.hbs

@@ -27,7 +27,7 @@
       {{/navbar}}
     </div>
     <div id="topbar-right" class="navbar-end">
-      <button class="navbar-burger" data-target="popup-menu">
+      <button class="navbar-burger" aria-controls="popup-menu" aria-expanded="false" aria-label="Toggle main menu">
         <span></span>
         <span></span>
         <span></span>