fix(auth): stop a config-only PUT deleting stored secrets (#2084)

Round 24 produced no inline comments again. Of the three suppressed items, one
is a pre-existing data-loss bug that predates #1950 entirely.

**Editing a server's config deletes its stored OAuth client secret — from the
keychain, not only from the stores this PR adds.** Filed as #2084 and fixed
here, since the review surfaced it and these lines were already changing.

`useServers.updateServer` deliberately sends `{ id, config }` with no
`settings`, so a config-only save cannot wipe persisted settings. The route
reads that as `preserve` and re-derives settings from the *on-disk* entry —
which by #1356's design no longer holds the secrets. The submitted secret set
is therefore empty, `expectedSecretFields` always lists the OAuth slot, and
the reconcile deleted a value the user never touched. The rename branch was
given `mergeRenameKeychainSecrets` for exactly this reason; the in-place
branch never got the equivalent.

The first fix was too broad — skip obsolete deletion whenever the intent is
`preserve` — and an existing test caught it immediately: stdio `env` is part
of `config`, so a config-only PUT that drops a key really has retired that
secret. Obsolete detection is per-field now:

- **`env:` fields** are implied by the entry's shape, so the new entry no
  longer expecting one is proof it was retired.
- **The OAuth slot** is implied by nothing, so absence counts as "cleared"
  only when the caller spoke about settings at all.

Both directions are tested, and the pre-existing env-key reconcile test still
passes.

Also:

- `refreshingPersist` moves to `lib/`. The combinator computes nothing, but
  what it exists to do is sequence side effects, which is the `lib` half of
  the repo's split; I judged it the other way on the grounds that the
  function is a pure higher-order one, and the rule is about what a module is
  *for*.
- `SecretStorageInfo.plaintext` is documented as file-only and *omitted*
  elsewhere rather than "always false", which is what the producer does and
  what `usableSecretStorage` enforces — the old wording invited a consumer to
  build the exact shape the web client discards.

Full `npm run ci` green, staged tree verified first.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/App.tsx

@@ -111,7 +111,7 @@ import { useMessageLog } from "@inspector/core/react/useMessageLog.js";
 import { useFetchRequestLog } from "@inspector/core/react/useFetchRequestLog.js";
 import { useStderrLog } from "@inspector/core/react/useStderrLog.js";
 import { useInitialConfig } from "@inspector/core/react/useInitialConfig.js";
-import { refreshingPersist } from "./utils/refreshingPersist";
+import { refreshingPersist } from "./lib/refreshingPersist";
 import { usePendingClientRequests } from "@inspector/core/react/usePendingClientRequests.js";
 import { InspectorView } from "./components/views/InspectorView/InspectorView";
 import type {