fix(inventory): make per-provider Coverage empty-state filter-aware (dont claim No usage detected when a chip filters all rows out)

[cristim]

Repro

1. Navigate to Inventory & Coverage > Coverage.
2. Select an Account or Provider chip that causes all rows in a specific provider's coverage box to be filtered out (e.g. select a provider like GCP when no GCP usage is in the current account chip selection).
3. The per-provider Coverage card shows "No usage detected for <Provider>".

Expected behavior

When a chip is active and filtered everything out, the message should be filter-aware: "No <Provider> usage for the selected account/filter" (or similar), making clear the result is filtered rather than genuinely absent.

When no chip is active and usage is truly absent, the original "No usage detected for <Provider>" message is correct.

Root cause

frontend/src/inventory.ts in buildProviderSection: the empty-state branch fires empty.textContent = \No usage detected for ${...}`wheneversection.services` is empty, with no awareness of an active account or provider chip.

The buildActiveCommitmentsEmptyMessage helper in the same file already implements the filter-aware pattern (branching on provider? / accountID?). The coverage empty-state branch should mirror that pattern.

PR #881 made filtering work but left this empty-state copy filter-unaware.

Note

Savings history chart already shows correct filter-aware empty state behavior.