Add human-readable output format with --output flag

- Improve output formatting with context-aware display for accounts,
  chats, messages, and search results
- Format timestamps as human-readable dates
- Truncate long strings for cleaner display
- Fix lint errors in tests (unchecked json.Encode returns)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: foeken