Bump ngx-echarts from 19.0.0 to 21.0.0 in /frontend (#709)

Bumps [ngx-echarts](https://github.com/xieziyu/ngx-echarts) from 19.0.0 to 21.0.0.
- [Release notes](https://github.com/xieziyu/ngx-echarts/releases)
- [Changelog](https://github.com/xieziyu/ngx-echarts/blob/master/CHANGELOG.md)
- [Commits](xieziyu/ngx-echarts@v19.0.0...v21.0.0)

---
updated-dependencies:
- dependency-name: ngx-echarts
  dependency-version: 21.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Author: dependabot[bot]

frontend/package-lock.json

@@ -28,7 +28,7 @@
     "colors": "^1.4.0",
     "dotenv": "^17.2.3",
     "echarts": "^6.0.0",
-    "ngx-echarts": "19.0.0",
+    "ngx-echarts": "21.0.0",
     "ol": "^10.6.1",
     "proj4": "^2.19.10",
     "resize-observer-polyfill": "^1.5.1",