Add route interactions: click-to-split, drag-to-reshape, colored route rendering

- Enrich BRouter response with per-point 3D coordinates and segment boundary
  tracking (EnrichedRoute interface)
- ColoredRoute component: plain, elevation gradient (green→yellow→red), and
  surface color modes with invisible wide polyline for click targeting
- Click-to-split: click on route polyline inserts waypoint at nearest point,
  mapped to correct segment via boundary indices
- MidpointHandles: draggable CircleMarkers at route segment midpoints for
  reshaping, hidden below zoom 12, opaque on hover
- Color mode toggle (select) synced via Yjs routeData
- i18n keys for color mode labels (en + de)
- Unit tests for segment boundary tracking (13 tests)
- E2E tests for enriched route response and color mode toggle

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/changes/planner-route-interactions/specs/map-display/spec.md

@@ -0,0 +1,20 @@
+## MODIFIED Requirements
+
+### Requirement: Route visualization
+The map SHALL display the computed route as an interactive, optionally color-coded polyline on the map.
+
+#### Scenario: Display route
+- **WHEN** BRouter returns a route GeoJSON
+- **THEN** the route is rendered as a polyline on the map, colored according to the active color mode
+
+#### Scenario: Route updates on waypoint change
+- **WHEN** a waypoint is added, moved, or deleted
+- **THEN** the route polyline updates after BRouter recomputes the route
+
+#### Scenario: Route is clickable
+- **WHEN** a user clicks on the route polyline
+- **THEN** a new waypoint is inserted at the clicked position (see route-splitting spec)
+
+#### Scenario: Route has midpoint handles
+- **WHEN** a route with two or more waypoints is displayed
+- **THEN** draggable midpoint handles appear on each route segment (see route-drag-reshape spec)