Slack install errors are now surfaced to the user

Hive’s Slack install flow now shows clear, specific error messages in the browser instead of failing silently. When a workspace admin tries to install the Slack app, validation of the OAuth callback now distinguishes between an expired install link, a user cancelled install, a Slack rejected install (with the Slack-provided reason), and a missing authorization code, surfacing each case as a flash alert on the install page. A new global flash component was added to render these messages consistently across the UI, and the README documents the Slack configuration steps and the callback URLs that the Slack app should be configured with (/slack/install/callback, /api/slack/events, /api/slack/interactions).