Split the editor sheet into two distinct elements:
- .editor-fab: floating action button, always a pill in the corner
- .editor-panel: sliding panel that animates in/out independently
This enables proper CSS keyframe animations (slide-up/down on mobile,
slide-in/out on desktop) with a closing class for exit transitions.
Simplified the JS hook to only handle close behaviour.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>