chore: add changesets for the macOS signing and server conflict fixes

Author: elkaix

.changeset/desktop-server-conflict.md

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+---
+'@pymodel/pythinker-code': patch
+---
+
+Say why the desktop app cannot start when another Pythinker server is already running. It now names the process, port and start time and offers Retry or Quit, in place of an exit code that explained nothing. Stopping the other server stays the user's choice.

.changeset/mac-dmg-signing.md

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+---
+'@pymodel/pythinker-code': patch
+---
+
+Sign, notarize and staple the macOS disk image, so a downloaded desktop build no longer opens with a Gatekeeper warning, and keep the update metadata in step with the finished file. The install window also gets a deliberate icon layout in place of the stock one.