Add initial visual snapshots and fix update script flag

Generate baseline screenshots (macOS/Chromium) for the elevation-chart
visual regression suite. The CI update-visual-snapshots workflow will
overwrite these with Linux variants on the first run.

Also fix the test:visual:update script: --update-snapshots is not a
valid Vitest 4 flag; the correct short form is -u.

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

apps/planner/package.json

@@ -11,7 +11,7 @@
     "lint": "eslint .",
     "test": "vitest run",
     "test:visual": "vitest run --config vitest.browser.config.ts",
-    "test:visual:update": "vitest run --config vitest.browser.config.ts --update-snapshots"
+    "test:visual:update": "vitest run --config vitest.browser.config.ts -u"
   },
   "dependencies": {
     "@codemirror/commands": "^6.10.3",