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.

api/dashboards/types.go

@@ -21,11 +21,15 @@ const (
 	VizBar        VizType = "bar"
 	VizSingleStat VizType = "single_stat"
 	VizTopN       VizType = "top_n"
+	// VizHeatmap is Phase 5's addition (log-volume-over-time patterns) --
+	// same "query already produced the right rows, only UI framing
+	// differs" shape as VizTopN, no new execution path.
+	VizHeatmap VizType = "heatmap"
 )
 
 func validVizType(v VizType) bool {
 	switch v {
-	case VizTable, VizLine, VizBar, VizSingleStat, VizTopN:
+	case VizTable, VizLine, VizBar, VizSingleStat, VizTopN, VizHeatmap:
 		return true
 	default:
 		return false
@@ -77,7 +81,7 @@ func validatePanel(p *Panel) error {
 		return fmt.Errorf("raw-SQL panels are not supported -- dashboards only support pipe-syntax queries, since the dashboard time-range picker is injected as leading query terms")
 	}
 	if !validVizType(p.VizType) {
-		return fmt.Errorf("viz_type must be one of table, line, bar, single_stat, top_n, got %q", p.VizType)
+		return fmt.Errorf("viz_type must be one of table, line, bar, single_stat, top_n, heatmap, got %q", p.VizType)
 	}
 	if len(p.VizConfig) == 0 {
 		p.VizConfig = json.RawMessage(`{}`)