u-link renders a native <a> inside its shadow root. aria-current or
aria-label set on the host element never reached that internal
anchor, since ARIA content attributes do not cross a shadow boundary
to a descendant — the host attribute was present, but the accessible
node actually exposed to assistive technology kept neither the
"current page" state nor a name. Navigation links marking the active
item with aria-current="page" were exposed with no current-page
indication at all.

render() now reads the host's aria-current/aria-label and binds them
directly onto the internal <a>. Neither is a Lit reactive property,
so neither was part of observedAttributes — attribute changes made
after the element connected were not observed at all, so a
static-only fix would have missed live updates (e.g. toggling the
active nav item). observedAttributes now includes both explicitly.

Also documents the three CSS custom properties (--error-icon-color,
--error-code-color, --error-message-color) of the router's built-in
fallback error page, previously undiscoverable — it renders whenever
no custom `fallback` is configured, but had no @cssprop tags or
reference doc entry.