feat: shortcuts settings, print dialog, slide reorder, presence sidebar (0.39.0)
Add reusable keyboard shortcuts settings panel UI with click-to-record
rebinding, conflict detection, localStorage persistence, and reset.
Sheets print: replace direct-print with options dialog. Users can now
choose page size, orientation, margins, scaling mode, grid lines, and
header repeat before printing or previewing.
Slides: add drag-to-reorder on thumbnail panel. Dragging a slide
thumbnail repositions it in the deck with visual drop indicators.
Collaboration: add presence sidebar showing who's online with
active/stale status dots, auto-refresh every 5s, and user avatars.
Closes #618, #619, #620, #622