Auto-insert intermediate nav-waypoints between selected waypoints

[msupino]

When the user builds a route by picking just the start and end (or by typing two distant codes via the multi-token search in #223), the planner could detect nav-waypoints that sit close to the great-circle line between them and offer to insert them.

Use case

CVFR flights in Israel hop through published reporting points. A pilot picking LLHZ → HADRA expects the route to thread through BAZRA, DEROR, SHARO automatically rather than having to type each one. This mirrors the way airline FMSes auto-fill enroute waypoints when given just an origin and destination.

Proposed behaviour

• When a leg is added (drop second waypoint, or multi-token search builds a route), scan navWP for points whose perpendicular distance to the leg's great-circle is below a small threshold (e.g. 2 NM) and whose along-track position is between the two endpoints
• Show those candidates in a small chip / dropdown next to the leg with an 'add' click — opt-in, not automatic, so the user keeps control over the route
• Optional: a modifier (e.g. Shift-click drop, or a toggle 'snap-route') that adds them all without confirmation

Scope

• Detection logic in interact.js or a new helper
• UI hook: chip / dropdown rendered next to the leg arrow (uses the existing drawLegArrow location)
• No new data: uses the already-loaded navWP array
• Should respect Show Navigation Waypoints toggle — if the layer is off, the user is explicitly ignoring nav-WPs and shouldn't be auto-suggested

Edge cases

• Very long leg (e.g. cross-country) with dozens of candidates — cap the list at a reasonable count
• User clicks somewhere off-route and types a custom name → don't trigger
• Reverse-route case: same logic, just the leg endpoints are reversed