fix(dashboard): merge per-service savings charts and populate current_savings (closes #908)

[cristim]

Summary

Closes #908. Merges the two Home per-service savings charts into one range chart with a current-savings underlay, and fixes the backend bug where per-service current_savings was never populated.

Backend bug fix

summarizeRecommendationsWithCoverage (internal/api/handler_dashboard.go) built by_service setting only PotentialSavings; ServiceSavings.CurrentSavings was always its float64 zero, so the frontend rendered an empty Current series.

Fix: also set svc.CurrentSavings += scaled, where scaled is the existing scaledSavings(rec, coverageByKey) value (rec.Savings * min(coverage,100)/100). This reuses the same coverage-scaled amount already used for the per-service potential, so current is keyed and scaled identically to potential. A regression test (TestSummarizeRecommendationsWithCoverage_PopulatesCurrentSavings) asserts non-zero, correctly-scaled current_savings per service; the existing table test now also asserts CurrentSavings.

Frontend merge

• Removed the standalone renderSavingsChart "Potential Savings by Service" chart and its #savings-chart-section / #savings-chart container in index.html.
• The surviving range chart (renderSavingsByService) now also draws each service's current (committed) savings as a separate bar in a DARKER shade of that service's potential hue. The darker shade is derived programmatically (darkenHexColor, ~30% per-channel reduction, hue preserved), not hardcoded per service.
• Preserved: top-N "+N more" cap, Provider/Account chip re-render (driven by loadDashboard), empty/zero states, non-empty axis + legend labels (a11y), chart instance dedup.

Tests

• go test ./internal/api/... green (1385 passing).
• Frontend jest green (2210 passing): merged chart renders the current underlay per service, current bar uses a darker variant, current defaults to 0 when a service is absent from by_service, chip-change re-render, empty-state, plus darkenHexColor / parseHexColor unit tests. html.test.ts asserts the old chart is gone.
• tsc --noEmit clean.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 17747f69-8a90-4df5-8401-29f4b47c4a71

📥 Commits

Reviewing files that changed from the base of the PR and between 80c20d2 and 5ccc301.

📒 Files selected for processing (6)

• frontend/src/__tests__/dashboard.test.ts
• frontend/src/__tests__/html.test.ts
• frontend/src/dashboard.ts
• frontend/src/index.html
• internal/api/handler_dashboard.go
• internal/api/handler_dashboard_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/908-merge-savings-chart

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