Fix leg marker zoom scaling + add magnifying glass tool

[msupino]

Two changes:

1. Leg marker zoom scaling: normalized inLabel/outLabel offsets to zoom-12 reference so kite markers stay proportional at every zoom level. Includes migration for existing saved routes.

2. Magnifying glass: circular toggleable magnifier (default 2x) with full editing support via pointer-events: none on the overlay — clicks pass through to the main map.

Branch: fix-leg-marker-zoom-scaling