Split ElevationChart + PlannerMap into deep modules; add visual regression testing

ElevationChart (1013 lines) split into:
- elevation-chart-draw.ts — pure drawElevationChart(ctx, w, h, params) function
- use-elevation-data.ts — useElevationData(routeData) hook
- ElevationChart.tsx — ~300 lines, interaction + render only

PlannerMap (869 lines) split into:
- MapHelpers.tsx — 7 Leaflet sub-components (MapExposer, RouteFitter, MapClickHandler,
  CursorTracker, NoGoAreaButton, OverlaySync, PoiRefresher)
- use-waypoint-manager.ts — all waypoint CRUD + route data sync
- use-gpx-drop.ts — GPX drag-and-drop hook
- PlannerMap.tsx — ~200 lines, orchestration only

Add Vitest browser visual regression tests for drawElevationChart via
@vitest/browser + Playwright (toMatchScreenshot). Tests cover plain, grade,
elevation, surface color modes plus hover and drag-select states.

Add update-visual-snapshots.yml workflow: triggered by workflow_dispatch or
the `update-snapshots` PR label; commits snapshots back to the branch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/planner/vitest.config.ts

@@ -12,5 +12,9 @@ export default mergeConfig(
         "~": resolve(import.meta.dirname, "app"),
       },
     },
+    test: {
+      // Exclude browser visual regression tests — they run via vitest.browser.config.ts
+      exclude: ["**/*.browser.test.{ts,tsx}"],
+    },
   }),
 );