Replace Chart.js with Apache ECharts in scoreboard charts

- Swap chart.js, chartjs-adapter-date-fns, chartjs-plugin-annotation, and
  chartjs-plugin-zoom for echarts v6 in the SPA dependencies.
- Reimplement PointsOverTimeChart and ChallengePlot against the echarts
  API (line charts, markLine boundaries, built-in dataZoom slider).
- Read axis, grid, legend, tooltip, slider, and data palette colors from
  Vuetify --v-theme-* tokens and re-render via a MutationObserver when
  the body theme class flips.
- Give .term-chart-wrap an explicit height and use a flex child so the
  chart container reports a non-zero size to echarts.init().
- Extend the x-axis range to include assignment boundary markers so the
  dashed guide lines stay inside the viewport.
- Rotate assignment labels 90 degrees and center them along each vertical
  boundary line; center the baseline label on the horizontal markLine.

Author: Nils Bars

docs/SCOREBOARD.md

@@ -172,10 +172,11 @@ endpoints and hands the data to the components under
   (regressions are filtered out).
 - `Countdown.vue` — timer for the currently-running assignment's deadline.
 
-All charts use Chart.js with `chartjs-plugin-zoom` for pan/zoom
-(drag-pan, wheel/pinch zoom, shift-drag box zoom) and cap the x-axis at
-the earliest data point so users can't drag into empty pre-data space.
-Chart data updates on each poll preserve the user's zoom state.
+All charts use Apache ECharts with native `dataZoom` on the time axis.
+The default interaction model is wheel/pinch zoom plus drag-to-pan on the
+x-axis, with a slider scrubber below the chart for coarse navigation.
+The x-axis is capped at the earliest data point so users can't pan into
+empty pre-data space.
 
 Badges are a visual consequence of crossing a scoring threshold — no
 dedicated backend. Badge assets are static SVG files at

spa-frontend/package-lock.json

@@ -11,11 +11,8 @@
   },
   "dependencies": {
     "@mdi/font": "^7.4.47",
-    "chart.js": "^4.4.7",
-    "chartjs-adapter-date-fns": "^3.0.0",
-    "chartjs-plugin-annotation": "^3.1.0",
-    "chartjs-plugin-zoom": "^2.2.0",
     "date-fns": "^4.1.0",
+    "echarts": "^6.0.0",
     "pinia": "^2.3.0",
     "vue": "^3.5.13",
     "vue-router": "^4.5.0",