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/specs/session-notes/spec.md

@@ -0,0 +1,12 @@
+## Requirements
+
+### Requirement: Collaborative session notes
+Planner sessions SHALL have a shared text area for participants to write notes.
+
+#### Scenario: Write notes
+- **WHEN** a user types in the notes area
+- **THEN** the text syncs in real-time to all other participants via Yjs
+
+#### Scenario: Notes persist
+- **WHEN** a user leaves and rejoins a session
+- **THEN** the notes are still there (stored in Yjs doc)