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/rate-limiting/spec.md

@@ -0,0 +1,15 @@
+## Requirements
+
+### Requirement: Session creation rate limit
+The Planner SHALL limit session creation to 10 per IP per hour.
+
+#### Scenario: Rate limit exceeded
+- **WHEN** an IP creates more than 10 sessions in one hour
+- **THEN** the server responds with 429 Too Many Requests
+
+### Requirement: BRouter call rate limit
+The Planner SHALL limit route computations to 60 per session per hour.
+
+#### Scenario: Routing rate limit exceeded
+- **WHEN** a session exceeds 60 BRouter calls in one hour
+- **THEN** the server responds with 429 and the client shows a "slow down" message