gitcode 0.4.1

Add a Codex-style startup update banner

gitcode now opens with an update picker when the published release is
newer than the installed one. The banner follows the Codex layout: a bold
headline with the version transition, a dim release-notes link, and three
numbered choices - update now, skip, or skip until the next version.

Lookups are cached in ~/.gitcode/version.json and refreshed in the
background every 20 hours, so startup never waits on the network and the
banner only ever shows a completed lookup. A skipped version stays hidden
until a newer one is published.