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/brouter-integration/spec.md

@@ -0,0 +1,20 @@
+## MODIFIED Requirements
+
+### Requirement: Route computation from waypoints
+The Planner SHALL compute a route between ordered waypoints by calling the BRouter HTTP API and returning the result as GeoJSON, preserving per-point elevation and segment boundary data.
+
+#### Scenario: Compute route with two waypoints
+- **WHEN** the routing host submits two waypoints (start, end) with profile "trekking"
+- **THEN** the BRouter API returns a GeoJSON route within 2 seconds
+
+#### Scenario: Compute route with via points
+- **WHEN** the routing host submits three or more waypoints
+- **THEN** the BRouter API returns a route passing through all waypoints in order
+
+#### Scenario: Per-point elevation preserved
+- **WHEN** BRouter returns GeoJSON with 3D coordinates [lon, lat, ele]
+- **THEN** the merged route response SHALL preserve elevation values for every coordinate point
+
+#### Scenario: Segment boundaries tracked
+- **WHEN** a route with N waypoints is computed (N-1 segments)
+- **THEN** the response SHALL include an array of coordinate indices marking where each waypoint-to-waypoint segment begins