Add global keyboard shortcuts with tooltip hints and a shortcuts sheet

One pure registry (apps/web/src/shortcuts.ts) defines each chord, its label,
and its platform-aware keycaps. Handlers, tooltip hints, sidebar row hints,
and the sheet all read from it. The listener loads after first paint and
ignores single keys while typing, prevented events, and open modals.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>