Adds a fullscreen button to the nav bar that expands the whole widget (viewer
+ nav bar), so the < > arrows and the dropdown stay visible and clickable in
fullscreen. Arrow KEYS (left/right) also navigate while the widget has focus
(container is made focusable and focused on entering fullscreen).
Navigation is already client-side (showIndex), so it works identically in
fullscreen with no re-execution. All listeners are on the pack's own
container element (never document), keeping it isolation-clean.
Caveat: when the mouse focus is inside the iframe 3D view, arrow KEYS go to
the iframe, not the container -- the on-screen arrow buttons always work; key
nav works when the nav bar/container holds focus. (Forwarding keys out of the
iframe would be a follow-up.)
PozzettiAndrea
78b0378f1d