Add release version checks and update command

[pmbstyle]

What changed

• add a release version check before octopal start so local and latest GitHub release versions are compared
• cache the release lookup and keep the check best-effort so startup stays fast and does not fail offline
• add uv run octopal update for safe git-checkout updates via git pull --ff-only and uv sync

Why

• now that GitHub releases exist, Octopal can proactively tell the user when their local checkout is behind
• a first-party update command makes the upgrade path much clearer from the CLI

Impact

• users get a lightweight upgrade prompt before startup when a newer release is available
• users on clean git checkouts can update in one CLI command
• background start avoids doing the version check twice

Validation

• uv run pytest tests/test_cli_update.py tests/test_cli_version_check.py tests/test_cli_webapp_build.py