v0.18.0

The uploader's empty state, rebuilt. One behaviour change to read before
upgrading: directories on box-drop-zone now adds a folder-browse control
rather than converting the file one, and box-drop-zone's internal markup
changed — a host reaching past part into its shadow DOM will need to look
again.

The uploader looks like an uploader

box-content-uploader's empty state was a small dashed box with two lines of
text in it. It now carries an illustration, the box-ui-elements wording, and
real browse controls, and it gained the action bar that library has had all
along.

A new upload-cloud illustration joins the design system: file cards fanned
behind a cloud carrying an upload arrow. It is registered art rather than markup
baked into the pattern, so a host that registers its own design system supplies
its own drawing and the uploader picks it up — and, since it is design-system
art, a page that registers no system at all still gets working copy and controls,
just no picture.

box-drop-zone gained variant="hero": the tall centred empty state, with an
illustration slot. variant="compact" stays the default and keeps the small
inline shape.

The browse affordance is now a real <button>. It was a <label> wrapping a
hidden input — clickable, but the control assistive technology saw was a file
input, and box-ui-elements has the same shape with a role="button" and a
hand-written keydown handler bolted on. A button needs none of that: correct
role, keyboard activation, and no nested-interactive ambiguity. Both inputs are
now tabindex="-1" and aria-hidden, because they are plumbing rather than the
control.

directories changed meaning. It used to convert the single input into a
folder picker, so browsing was files or folders. It now adds a second
control with its own webkitdirectory input, so both are offered — which is
what box-ui-elements does. The 0.17.0 note claiming the platform forces the
choice was wrong: it forces it per input, not per drop zone.

box-content-uploader gained a Close / Cancel / Upload bar, always visible,
with controls disabled rather than absent — a bar that materialises under the
pointer as the first file lands is how people click the wrong thing. Upload
starts the queue (which matters with auto-start="false"), Cancel stops
everything still in flight, and Close emits a cancelable close event: the
uploader does not own the surface it sits in, so the host decides what closing
means. closable="false" hides it for hosts with their own dismissal.

Close is held while uploads are running. box-ui-elements disables it for any
non-empty queue, which leaves a person on a finished queue with no way out, so
here it is only held while something is actually in flight.

The bar wraps rather than squeezes. Five controls and a status line do not fit a
narrow column on one row: Close and the status keep the first row, and the
action group drops to its own row, still right-aligned, with labels intact.

The empty state and the queue are now alternatives rather than neighbours: once
files are queued the illustration gives way to the list, as it does upstream.
Dropping still works over the list.