Savings History KPI units wrong ($/hr vs $/mo) + add Hourly/Monthly/Yearly dropdown #750

Symptom

On the Purchases page "Savings History" widget, the three KPIs display with inconsistent and seemingly-wrong units:

• Period Savings: $1.15 (no unit suffix)
• Avg Hourly Savings: $1.15/hr
• Peak Savings: $1.15/hr

Two issues:

1. Peak Savings is labeled /hr but the underlying value is monthly. With the single committed purchase being a t4g.nano AWS ec2 reservation, the monthly saving is ~$1.15/month. $1.15/hr would be ~$840/month -- not what's actually happening. So the unit label is wrong.
2. Avg Hourly Savings is also labeled /hr but is likely the same monthly number. Same coincidence of value as Peak.

Feature ask

Add a dropdown on the Savings History widget to switch the view between Hourly / Monthly / Yearly, and have all three KPIs (Period / Avg / Peak) re-render in the selected unit.

Acceptance criteria

• Peak Savings shows the correct unit suffix matching the underlying value (likely /mo for the current data).
• Avg savings shows the correct unit suffix matching the underlying value.
• A dropdown lets the user switch all three KPIs to Hourly / Monthly / Yearly consistently.
• The chart period selector (currently 7d / 30d / 90d / All) is independent of this unit selector.

Files likely involved

• frontend/src/modules/savings-history.ts (KPI rendering + period selector)
• internal/api/analytics_postgres.go or wherever the Period / Avg / Peak values are computed -- confirm whether the API already returns multiple unit variants or only one.

Source

QA exploratory testing, 2026-05-27.