Sticky Day Headers, Email Failure Reasons, and Bulk Copy

Added sticky day header in chat messages (Slack-like TODAY/YESTERDAY pin)

Added day view to Signal schedule calendar for partners with many locations

Added bulk copy for URLs and conversation IDs (Cmd+K on selected rows)

Added email failure reason in dispatch status tooltip (Sendgrid failure data in custom attributes)

Fixed failed-to-parse AI response in generateConversationSummary