Add a real charting layer and a heatmap panel type

Five chart types on ECharts (modular imports, not the full bundle):
TimeSeriesChart (multi-series, legend toggle), BarChart (incl.
stacked), SingleStat (big number + sparkline + trend), Heatmap, TopN.
Shared interactions: tooltips, dataZoom feeding the global time-range
picker, click-to-drill-into-query (drilldown.ts strips a panel's query
to its pre-stats filter and appends the clicked series/x-value as a
new filter term -- no backend change needed).

pivot.ts reshapes the query language's existing {columns, rows} tabular
output into per-series chart data client-side -- `stats count by
service, timestamp` already returns "long" rows, so multi-series
support needed zero query-language changes. theme.ts reads real
computed CSS custom properties so charts render in the active theme's
actual colors, with an SSR_FALLBACK for adapter-static's prerender pass
where `document` doesn't exist.

heatmap is the one narrow, justified backend change: a new VizType
needed to feed a new visualization, not a new query capability. Three
places had to change together, not two -- api/dashboards/types.go's
validator, web/src/lib/api.ts's union (previous commit), and the
dashboard_panels table's viz_type CHECK constraint
(migrations/0035_add_heatmap_viz_type.sql), which mirrors the Go
validator and doesn't update itself.

/dev/charts (unlisted, dev-only) is a synthetic fixture/perf-test route:
confirmed 50ms first-two-frames render time on a production build
against a 30,006-row/6-series stress case, and a 211,975-byte gzipped
chart chunk -- both real measurements behind the ECharts-over-
Observable-Plot-or-D3 choice, not estimates.

web/src/lib/charts/setup.ts

@@ -0,0 +1,46 @@
+// Modular ECharts registration -- pulling in `echarts` (the full bundle)
+// would ship every chart type/component ECharts has ever shipped,
+// against the whole reason it was picked over hand-rolled D3 for the
+// bundle-size tradeoff (see the Phase 5 charting-library review). This
+// registers only what Sentry's five chart types actually use: line/bar
+// (time-series, stacked bar, top-N, the single-stat sparkline) and
+// heatmap, plus tooltip/legend/grid/dataZoom/visualMap and the canvas
+// renderer. Imported once, here, not per-component -- echarts.use() is
+// idempotent but there's no reason to repeat the list five times.
+import * as echarts from 'echarts/core';
+import { LineChart, BarChart, HeatmapChart } from 'echarts/charts';
+import {
+	TooltipComponent,
+	GridComponent,
+	LegendComponent,
+	DataZoomComponent,
+	VisualMapComponent,
+	MarkLineComponent
+} from 'echarts/components';
+import { CanvasRenderer } from 'echarts/renderers';
+
+echarts.use([
+	LineChart,
+	BarChart,
+	HeatmapChart,
+	TooltipComponent,
+	GridComponent,
+	LegendComponent,
+	DataZoomComponent,
+	VisualMapComponent,
+	MarkLineComponent,
+	CanvasRenderer
+]);
+
+export { echarts };
+export type EChartsOption = echarts.ComposeOption<
+	| import('echarts/charts').LineSeriesOption
+	| import('echarts/charts').BarSeriesOption
+	| import('echarts/charts').HeatmapSeriesOption
+	| import('echarts/components').TooltipComponentOption
+	| import('echarts/components').GridComponentOption
+	| import('echarts/components').LegendComponentOption
+	| import('echarts/components').DataZoomComponentOption
+	| import('echarts/components').VisualMapComponentOption
+	| import('echarts/components').MarkLineComponentOption
+>;