fix(web): finish Codex sign-in when the tab becomes visible again (#214)

## Related Issue

No issue; found while auditing the browser sign-in flow.

## Problem

The Codex sign-in composable polls the login status on an interval and
immediately when the window regains focus. A tab that was hidden and
then shown again without a `focus` event (mobile tab switch, some
desktop window managers, background tabs restored by the browser) kept
waiting for the next interval tick before it noticed the completed
sign-in.

## What changed

- `useCodexLogin.ts`: also poll on `document` `visibilitychange` when
the document is visible; remove the listener on unmount alongside the
focus listener.
- `use-codex-login.test.ts`: new case proving a hidden→visible
transition polls once and a hidden transition does not.
- `apps/pythinker-code/dist-web`: rebuilt from source (`pnpm run
build:web`, `check-web-assets` OK). The chunk churn is hash renames
only.

## Verification

- `pnpm --filter @pymodel/pythinker-web exec vitest run` — 84 files,
1,000 tests passed
- `pnpm --filter @pymodel/pythinker-web typecheck`, `check:style` — exit
0
- `pnpm run build:web` + `node
apps/pythinker-code/scripts/check-web-assets.mjs` — Web assets OK

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [x] I have linked a related issue (external PRs: the issue must have a
maintainer's `/approve`).
- [x] I have added tests that prove my feature works.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Codex sign-in now completes automatically when you return to the
browser tab, in addition to restoring window focus.
* Updated web assets include expanded diagram, chart, language, and
editor support.
* **Bug Fixes**
* Improved login-status detection after switching back to the
application.
* **Tests**
* Added coverage for checking login status when the tab becomes visible.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix

.changeset/codex-login-visibility-wake.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Complete a Codex sign-in as soon as the browser tab becomes visible again, not only when the window regains focus.

apps/pythinker-code/dist-web/.web-bundle-manifest.json

@@ -1,4 +1,4 @@
 {
-  "sourceHash": "ad0be8811299a96afcfa243941b6812309cf4597345759fac292edbcf7dea1e4",
+  "sourceHash": "c05b9ce73324f5a446966d9f931dc804f0f1e73e78daf14e1479d86e58669c1a",
   "sourceFileCount": 404
 }