Fix Jupyter notebook keyboard input handling

[brendancol]

Summary

• Use data-lm-suppress-shortcuts attribute to prevent JupyterLab/Lumino from intercepting keyboard shortcuts on the viewer widget, instead of stopPropagation() which blocked ipyevents from receiving key events
• Disable/enable Classic Notebook keyboard manager on focus/blur to prevent notebook shortcuts (e.g. x deleting cells) from firing while the viewer is focused
• Suppress escape and x exit keys in Jupyter since they're too easy to hit accidentally — use widget.stop() instead
• Set ipyevents wait=0 for lower-latency keyboard/mouse input
• Consolidate startup print into a single line

Test plan

• Open explore() viewer in JupyterLab — verify WASD/arrow keys, R/Shift+R, H, and mouse controls all work without triggering notebook shortcuts
• Verify clicking the image shows blue focus border, clicking outside removes it
• Verify x and escape do not close the viewer
• Verify widget.stop() still works to close the viewer
• Test in Classic Notebook if available — verify keyboard manager is disabled/enabled on focus/blur