Add per-service savings-range bar chart on Home page (solid floor + lighter range, surface ByService data already on the wire) #765

Summary

Add a per-service savings-range bar chart to the Home page, supplementing the existing cumulative savings-over-time line chart. The backend already exposes the necessary data via HistoryDataPoint.ByService map[string]float64 on /api/dashboard/savings-over-time; the frontend currently drops that field on the floor.

Design

• Layout: one vertical bar per service (EC2/RDS/ElastiCache/OpenSearch/Compute/Database/Cache/... — whatever services appear in the response over the selected timeframe).
• Bar composition: the bar runs from 0 to max(savings) for that service across the selected timeframe, split into two shades:
  • Solid base from 0 to min(savings) for that service — the "guaranteed floor": this service has consistently delivered at least this much across the window.
  • Lighter shade from min to max(savings) — the "range upside": how much the savings can swing over the window.
• Y-axis: dollars (same units as the existing chart).
• X-axis: service names (sorted by max desc so the largest service is leftmost).
• Hover tooltip: per service, show min / median / max / count-of-points, and the percentage of total realized savings the service contributed.
• Empty state: when no service has positive savings in the window, mirror the line chart's friendly empty-state message.
• Filter coupling: the new bar chart honours the same account / provider / timeframe filter chips as the line chart and KPI cards (see Home chart drifts from KPIs when a provider filter is active (analytics endpoint missing provider param) #764 for the provider filter gap that affects all three).

Data path

HistoryDataPoint.ByService is already returned per data point by the analytics handler. Frontend just needs to:

1. Iterate the data_points array, accumulate per-service min/max/sum/count over the window.
2. Render two-stacked bars: [0 -> min] solid, [min -> max] light.

No backend change required unless by_service is missing on some data points (in which case the analytics aggregator needs to populate it consistently — verify).

Placement

Below the existing cumulative line chart in the Home page Savings section. Same width, same range-selector controls (1d/7d/30d/90d/all) reused.

Test plan

• Unit test: synthetic data_points with known per-service values produces correct min/max bars.
• Empty state when zero positive-savings services.
• Multi-account filter shrinks bars to the filtered subset.
• Hover tooltip values match the bar geometry.
• Regression: existing cumulative chart still renders correctly above the new bars.

Out of scope (separate issues)

• Provider filter on the analytics endpoint — tracked by Home chart drifts from KPIs when a provider filter is active (analytics endpoint missing provider param) #764.
• Replacing the cumulative line chart entirely — decide after this lands, based on UX feedback.