feat: improve doctor command logging and automatic server capability detection

- Replace SimpleLogger with structured logging using slog and tint library
- Add colorized output with proper log levels (DEBUG/INFO/WARN/ERROR)
- Remove manual DetectServerCapabilities() calls from doctor command
- Server capabilities are now automatically detected on client connection
- Update default test credentials to admin@localhost/admin for consistency
- Improve testXEP0077 to use fail-fast approach with proper ghost user workflow
- Add structured logging throughout all test functions with timing information

🤖 Generated with [Claude Code](https://claude.ai/code)

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

go.sum

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