feat: show redirect URL in forge connection form
Display the OAuth callback URL as a read-only field so users know what
to configure in their forge's OAuth application settings. Extract
redirect_url/0 in Sower.Forge.Oauth to avoid duplicating the URL
construction.
sow-114
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>