fix(settings): stop the infra-billing page rendering permanently in deselected-sites state

[pedrofrxncx]

Follows up on the recently-added infra-billing UI (apps/web/src/views/settings/infra-billing.tsx, apps/web/src/hooks/use-infra-billing.ts).

The bug: useInfraBilling's query is enabled: slugs.length > 0. When a user deselects every site in the MultiSelect, narrowedTo becomes [] (not null), so selected is [] and the query stays disabled forever — data never resolves and isLoading never becomes true. Two visible consequences on the page:

1. The billing summary card falls into the !isLoading branch and renders billingMessage, which defaults to the "multiple_teams" key when data?.billingUnavailableReason is undefined — showing a wrong "sites belong to different legacy teams" message even though the real cause is just an empty selection.
2. The invoices table's !data check renders a <Skeleton> that never resolves — a permanent loading spinner.

The fix: skip the summary/metrics/invoices SettingsSections entirely when selected.length === 0 — the existing pickASite card above already covers that state, so there's nothing else to show.

Reviewer check: open Settings > Infra Billing with an org that owns 2+ sites, then clear the site MultiSelect down to zero selections — confirm only the "pick a site" card shows, with no stuck skeleton or wrong billing message underneath.

Verified locally: bun run fmt, cd apps/web && bunx tsc --noEmit (green), bunx oxlint apps/web/src/views/settings/infra-billing.tsx (0 warnings/errors). No existing unit test covers this React view; full CI validates the rest.

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Summary by cubic

Prevents stuck loading and wrong messaging on Infra Billing when the site selection is empty. Previously, deselecting all sites left the query disabled, showed a misleading “multiple teams” message, and kept the invoices table skeleton visible; now the summary, metrics, and invoices sections are hidden when no sites are selected so only the “pick a site” card remains.

• Reviewer notes:
  • In Settings > Infra Billing, clear the site MultiSelect to zero selections. Confirm only the “pick a site” card shows, with no skeletons or billing message underneath.
  • Change is isolated to apps/web/src/views/settings/infra-billing.tsx: the summary/metrics/invoices SettingsSections render only when selected.length > 0.

^{Written for commit 585f89a. Summary will update on new commits.}