[LHagfoss]

What

Check all marked harnesses as failing that pass when checked one by one. It started every check at once, and every answer replaced the whole harness snapshot, so a late, older answer brought back a stale detection.

• A harness snapshot is applied only if no later request's answer has been applied already (a request sequence number shared by refreshHarnesses and harnessCall).
• Check all sends one account/check that re-detects every harness (one plan/Keychain read), then starts the harnesses two at a time to list their models. A harness already being checked is joined.
• The Check all button reads "Checking 2 of 5…" while it runs.
• Failures already show the real error text on the card; unchanged.

Changelog

Check all in Settings > Harnesses now ends with the same results as pressing Check on each harness, and shows its progress.

Testing

• bun run check in apps/desktop (typecheck, lint, tests, build).
• New tests in harnessChecks.test.ts: Check all ends in the same per-card results as checking each alone; one account/check with every id; at most two model listings at once; joining a running check; a delayed older snapshot does not replace a newer one (fails without the guard).

Closes #3448