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>

.github/workflows/update-visual-snapshots.yml

@@ -0,0 +1,103 @@
+name: Update visual snapshots
+
+# How to use this workflow
+# ========================
+#
+# Visual snapshots live in `apps/planner/app/**/__screenshots__/` and are
+# committed to the repo. They are generated by Vitest browser mode running
+# the Planner's `*.browser.test.tsx` files against real Chromium via Playwright.
+#
+# When to update snapshots:
+#   - You intentionally changed the look of the elevation chart (new color mode,
+#     layout change, etc.) and the old snapshots are now wrong.
+#   - You added a new `*.browser.test.tsx` test and need the initial snapshots.
+#
+# Two ways to trigger this workflow:
+#
+#   1. Manual dispatch (workflow_dispatch):
+#      Go to Actions → "Update visual snapshots" → "Run workflow".
+#      Choose the branch you want updated. The workflow will commit the new
+#      snapshots back to that branch.
+#
+#   2. PR label (update-snapshots):
+#      Add the `update-snapshots` label to any PR. The workflow will update
+#      snapshots on the PR's head branch. Remove the label after to avoid
+#      re-triggering on every subsequent push.
+#
+# After the workflow commits updated snapshots, pull the branch locally:
+#   git pull origin <your-branch>
+#
+# Running locally:
+#   pnpm --filter @trails-cool/planner test:visual           # run tests
+#   pnpm --filter @trails-cool/planner test:visual:update    # update snapshots
+#
+# Platform note:
+#   Snapshots are generated on ubuntu-latest to keep CI and local results
+#   consistent. Snapshots generated on macOS or Windows will have subtle
+#   font-rendering differences and will fail on CI. Always use this workflow
+#   (or a Linux machine / Docker) to produce the canonical snapshots.
+
+on:
+  workflow_dispatch:
+    inputs:
+      branch:
+        description: "Branch to update snapshots on"
+        required: false
+        default: ""
+
+  pull_request:
+    types: [labeled]
+
+jobs:
+  update-snapshots:
+    # Only run for manual dispatch, or when the label is "update-snapshots"
+    if: >
+      github.event_name == 'workflow_dispatch' ||
+      (github.event_name == 'pull_request' && github.event.label.name == 'update-snapshots')
+
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: write  # needed to push snapshot commits back
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.ref || github.event.inputs.branch || github.ref }}
+          # Use a token with push rights so the commit-back step can push
+          token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: ".nvmrc"
+          cache: "pnpm"
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Install Playwright Chromium
+        run: pnpm exec playwright install chromium --with-deps
+
+      - name: Update visual snapshots
+        run: pnpm --filter @trails-cool/planner test:visual:update
+
+      - name: Commit updated snapshots
+        uses: stefanzweifel/git-auto-commit-action@v5
+        with:
+          commit_message: "chore: update visual snapshots [skip ci]"
+          file_pattern: "apps/planner/app/**/__screenshots__/**"
+          commit_user_name: "github-actions[bot]"
+          commit_user_email: "github-actions[bot]@users.noreply.github.com"
+
+      - name: Upload snapshots as artifact
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: visual-snapshots
+          path: apps/planner/app/**/__screenshots__/
+          if-no-files-found: ignore