chore(desktop): prepare 0.3.1 release (#190)

## Related Issue

No tracking issue — this is maintainer-side recovery for the unpublished
desktop `v0.3.0` draft.

## Problem

The immutable `desktop-v0.3.0` tag points to the release commit whose
Windows build failed. The corrected workflow is now on `main`, but the
desktop version remains `0.3.0`, so release automation cannot cut a new
tag.

## What changed

- Add a confirmed patch changeset for `@pymodel/pythinker-desktop`.
- Schedule desktop `0.3.1` so the Changesets version PR can create a new
immutable release boundary.

This PR does not change application code, move the failed tag, or
publish a release. Merging the generated version PR remains a separate,
explicit publication decision.

## Verification

- `changeset status` — only `@pymodel/pythinker-desktop` receives a
patch bump.
- Major changeset policy self-test — 19 cases passed.
- Major changeset policy against `origin/main` — no new major changeset.
- Pre-push-equivalent gates — Sherif, lint with 0 errors, web assets,
Nix hash, no affected typecheck packages, and no affected tests.
- Internal-identifier audit — no findings.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [x] No related issue is required for this maintainer-directed release
recovery.
- [x] Tests are not required for changeset-only release metadata; the
affected-test gate found no tests.
- [x] Ran `gen-changesets`; this PR contains the confirmed desktop patch
changeset.
- [x] Ran `gen-docs`; no documentation update is required.

Author: elkaix

.changeset/restore-desktop-downloads.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-desktop": patch
+---
+
+Restore downloadable desktop releases for macOS and Windows.