feat(history/analytics): honour provider query param for Savings-over-time chart

[cristim]

Background

Discovered while fixing #498: the frontend's loadSavingsTrendChart()
reads state.getCurrentProvider() but does NOT pass it to
getSavingsAnalytics(...) because the backend handler
(internal/api/handler_analytics.go::getHistoryAnalytics) does not
honour a provider query param — it only takes account_id
(documented as an Athena limitation).

As a result, switching the Provider chip on the Home page does not
truly filter the Savings-over-time chart. What happens today: the
topbar-filters.ts provider-change handler clears the account
selection (per the #185 ordering rule), which fires the account
subscriber, which triggers loadDashboard() → loadSavingsTrendChart()
with no account_ids → backend returns "all accessible accounts".
The chart shape changes (because we reset to All) but isn't actually
provider-filtered.

The user report in #498 captured this as the "Provider-filter caveat"
(row 2.2): "graph adjusts. Can't confirm if data itself is correct."

What to do

Wire provider through the analytics handler:

1. internal/api/handler_analytics.go::getHistoryAnalytics reads
   provider from params (currently only reads account_id and
   interval).
2. Pass provider to analyticsClient.QueryHistory(...) — extend the
   client signature if needed.
3. The Postgres query in analyticsClient.QueryHistory filters by
   provider when present (additive to the existing account_id
   filter).
4. Frontend (dashboard.ts::loadSavingsTrendChart +
   modules/savings-history.ts::loadSavingsHistory) starts passing
   provider: state.getCurrentProvider() to getSavingsAnalytics.
5. Regression test: a request with provider=aws&account_id= returns
   only AWS rows; a request with provider=aws&account_id=acct-X
   returns only AWS rows for acct-X.

Acceptance criteria

• Backend /history/analytics?provider=aws returns AWS-only data
  points; ?provider=gcp returns GCP-only.
• Provider chip change on the Home page produces visibly different
  chart data when the underlying analytics history contains rows from
  multiple providers.
• Existing single-account-only queries continue to work (backward
  compatible).

[cristim]

Verification: shipped in PR #716 (SHA fd33b50, merged 2026-05-25). handler_history.go reads provider from query params, passes it via HistoryFilter.Provider. Confirmed present on origin/feat/multicloud-web-frontend.