Closes the "no keyboard shortcut help dialog" audit gap (MEDIUM).
Changes:
- Add KeyboardShortcutsDialog component: portal-based, accessible
dialog listing all canvas + navigation + agent shortcuts grouped by
category. WCAG 2.1 compliant (focus trap, Esc close, aria-modal,
aria-labelledby, focus restoration on close).
- Add global ? shortcut: opens the dialog when pressed outside any
input field and no modal is already open.
- Add "See all shortcuts →" link in the Toolbar quick-start popup
linking to the dialog.
Test plan:
- [x] npx vitest run (182 tests pass)
- [x] tsc --noEmit (no type errors)
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
