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/route-splitting/spec.md

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+## ADDED Requirements
+
+### Requirement: Insert waypoint by clicking on route
+The Planner SHALL allow users to insert a new waypoint by clicking on the rendered route polyline.
+
+#### Scenario: Click on route between two waypoints
+- **WHEN** a user clicks on the route polyline between waypoint 2 and waypoint 3
+- **THEN** a new waypoint is inserted at position 3 (between the original waypoints 2 and 3) at the clicked location, and the route recomputes
+
+#### Scenario: Waypoint snaps to route
+- **WHEN** a user clicks near the route polyline
+- **THEN** the new waypoint is placed at the closest point on the existing route geometry, not at the raw click position
+
+#### Scenario: Split syncs to other participants
+- **WHEN** a user inserts a waypoint by clicking on the route
+- **THEN** all other participants see the new waypoint appear in the waypoint list and on the map via Yjs sync