On startup, if `update_check_interval_hours` have elapsed since the last
check, fetches the latest release tag from the Gitea API and prints a
notice when a newer version is available. Silent on network failure or
no update found.
- New `update_check` module with injected fetcher for full testability
- Cache at ~/.cache/tmuxido/update_check.json tracks timestamp + version
- `fetch_latest_tag` and `version_compare` promoted to `pub(crate)`
- 6 unit tests covering disabled, interval not elapsed, fetch triggered,
equal versions, update available, and current-newer edge case
