test(web): add unit tests for usage format helpers

[dylantirandaz]

What this changes

Adds one colocated test file, apps/web/src/features/usage/format.test.ts, covering the previously untested pure formatter module apps/web/src/features/usage/format.ts. No source changes — this PR adds tests for previously untested behavior only.

Coverage pins the exact output strings of the observable contract:

• formatDuration: 'a few seconds' below 5s, negative clamping, pluralization, and the s → m → h → day rounding boundaries (59s/60s, 59m/60m, 47h/48h → 2 days).
• ageLabel: future timestamps ('in X'), the ±5s 'just now' window, and past 'X ago'.
• usageAmountLabel: used/limit vs remaining-only vs fraction-only rendering, the 'No amount reported' fallback, percent precision (2 decimals below 1%, 1 decimal at/above), byte thresholds at exact GiB/MiB/KiB/B boundaries, USD formatting, and unit suffixes (tokens/requests/min).
• limitDisplayName: tier/window suffix appending, case-insensitive dedup, and 'quota window' suppression.
• resetLabel: null cases, passed resets, future countdown.
• capacityLabel: account pluralization.
• savedResetLabel: null/zero cases, pluralization, expiry parsing with unparseable/past/due-now filtering, sort order, and the 'next expires in X' suffix.
• reportIdentityDetail: org/plan composition and account-label dedup.

Verfication

• pnpm --filter @t4-code/web test — 72 files, 1029 tests passed (includes the new file).
• pnpm check — passes (release contract, provenance, lint --deny-warnings, typecheck).
• Did not run the full repo pnpm test; per project notes, the pre-existing packages/client transcript-retention test failure on main is unrelated to this change.

Checklist

• pnpm check passes (release contract, provenance, lint, typecheck)
• pnpm test passes; new behavior has a test that fails without this change (this PR is the tests; no source behavior changed)
• Any pasted logs or screenshots are redacted: no secrets, tokens, pairing codes, or private paths
• Ported code (T3 Code / OMP) follows the provenance rules in THIRD_PARTY_NOTICES.md