Surface Slack install failures with clear flash messages

When a Slack workspace install fails, Hive now shows a clear flash message instead of failing silently. The Slack OAuth callback now distinguishes between expired install links, denial by the user, errors returned by Slack, and missing authorization codes, and surfaces the specific reason (“The Slack install link expired. Try again.”, “Slack install was cancelled.”, “Slack rejected the install: .”). As part of the same change, a shared flash_group component and flash stack styling were added to the root layout so flash messages render as dismissible alerts site-wide.