fix(auth): keep oauth-persist browser-safe (move Node file backend out)

The refactor routed the browser's RemoteOAuthStorage through
`oauth-persist.ts`, which value-imported the Node-only `store-io.js`
(`atomically` -> `stubborn-fs` -> `node:process.getuid`, plus
`node:fs/promises`). That dragged Node built-ins into the browser bundle
and the web app crashed to a blank page at runtime — a regression from
v2/main, where browser OAuth storage used a Zustand JSON adapter with no
Node imports. Unit/integration tests run in node/happy-dom and
`smoke:web` only checks the served HTML, so none of them execute the app
in a real browser and the break slipped through.

Fix, following the existing `store-id.ts` precedent:
- Split the pure JSON (de)serializers into Node-free
  `core/storage/store-serialize.ts`; `store-io.ts` re-exports them so
  existing importers are unaffected.
- Move the Node-only `createFileOAuthPersistBackend` into
  `core/auth/node/oauth-persist-file.ts`; update `storage-node.ts` and
  the storage adapters test to import from there.
- `oauth-persist.ts` now imports only the Node-free serializers, so the
  browser bundle no longer pulls in `node:fs`/`atomically`.

Tests:
- oauth-persist-browser-safe.test.ts: source-level guard that
  `oauth-persist.ts` never imports store-io/node:/atomically.
- Direct unit tests for the isomorphic remote + session backends
  (restoring oauth-persist.ts coverage after the file backend moved out).
- store.test.ts: OAuthMemoryStore unit coverage incl. replace() fallbacks.
- storage.test.ts: clearAllOAuthClientState null-snapshot (no file) path.

Verified end-to-end in a real browser (Playwright): DCR OAuth flow
completes, tokens persist via the remote store, and after a full page
reload the server reconnects with no re-authentication.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/web/src/test/integration/auth/node/storage.test.ts

@@ -579,6 +579,18 @@ describe("NodeOAuthStorage with custom storagePath", () => {
     resetNodeOAuthStorageCache();
   });
 
+  it("clearAllOAuthClientState is a no-op when no store file exists", async () => {
+    // Exercises the null-snapshot branch: read() returns null, so
+    // `snapshot?.servers ?? {}` yields no urls and nothing is cleared.
+    const filePath = getStateFilePath();
+    resetNodeOAuthStorageCache();
+    await fs.unlink(filePath).catch(() => {});
+
+    await expect(clearAllOAuthClientState()).resolves.toBeUndefined();
+
+    resetNodeOAuthStorageCache();
+  });
+
   it("should isolate state from default store", async () => {
     const customPath = path.join(
       os.tmpdir(),