Replace unreliable window event listener with transparent full-screen
backdrop element. Clicking anywhere outside the radial menu now properly
dismisses it. Also handles right-click on backdrop for dismiss.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Implements RadialMenu component (general-purpose, configurable wedges)
and useContextMenu hook (right-click on desktop, 450ms long-press with
8px movement threshold on touch).
First wired action: "What's here" — reverse-geocodes the trigger
location and opens the place panel for the result. Remaining wedges
(Drop pin, Directions from here, Directions to here, Add as stop,
Save place) render but stub to a toast — wiring deferred to follow-up
sessions.
Per design doc NAVI-DIRECTIONS-REDESIGN.md sections covering Phases a
and b of the implementation sequence.
