web: reach-over-time chart with playhead, slice 5 of M3

Hand-rolled SVG as decided: dashed percent grid, one cumulative curve
per panel in its accent color, end labels via the shared formatPct, and
a playhead line with per-curve dots synced to the global round. The
cumulative-at-round counting moved into lib/reach.ts so the panel stats
and the chart cannot disagree.

web/src/App.vue

@@ -7,6 +7,7 @@ import HeroHeadline from '@/components/HeroHeadline.vue'
 import LegendRow from '@/components/LegendRow.vue'
 import PanelGrid from '@/components/PanelGrid.vue'
 import PlayerBar from '@/components/PlayerBar.vue'
+import ReachChart from '@/components/ReachChart.vue'
 import ResultsTable from '@/components/ResultsTable.vue'
 import ScenarioToolbar from '@/components/ScenarioToolbar.vue'
 import { usePlayback } from '@/composables/usePlayback'
@@ -32,6 +33,7 @@ onMounted(async () => {
     <PanelGrid />
     <LegendRow />
     <PlayerBar />
+    <ReachChart />
     <ResultsTable
       :panels="store.state.panels"
       :results-by-panel-id="store.state.resultsByPanelId"