Replace logging system with slog and tint for structured colored output

- Add logger.go with public Logger variable and InitLogger function
- Replace all fmt.Printf/fmt.Fprintf calls with structured Logger.Info/Logger.Error
- Initialize logger in main function for consistent access across packages
- Keep fmt.Errorf for proper error creation (standard Go practice)
- Add tint dependency for colorized terminal output with timestamps
- Convert user output to structured logging with key-value pairs
- Update info command to use structured logging for plugin details
- Update updateassets command to use structured progress logging
- Update version command to use structured logging
- Update authentication logging in client.go with structured fields
- Update enable/disable commands to use structured logging
- Remove unused fmt imports after conversion

All output now uses slog with tint for beautiful, structured, colorized logging
while maintaining proper error handling with fmt.Errorf for error creation.

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

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

go.sum

@@ -870,6 +870,8 @@ github.com/leonklingele/grouper v1.1.2 h1:o1ARBDLOmmasUaNDesWqWCIFH3u7hoFlM84Yrj
 github.com/leonklingele/grouper v1.1.2/go.mod h1:6D0M/HVkhs2yRKRFZUoGjeDy7EZTfFBE9gl4kjmIGkA=
 github.com/letsencrypt/boulder v0.0.0-20250411005613-d800055fe666 h1:ndfLOJNaxu0fX358UKxtq2bU8IMASWi87Hn0Nv/TIoY=
 github.com/letsencrypt/boulder v0.0.0-20250411005613-d800055fe666/go.mod h1:WGXwLq/jKt0kng727wv6a0h0q7TVC+MwS2S75rcqL+4=
+github.com/lmittmann/tint v1.1.2 h1:2CQzrL6rslrsyjqLDwD11bZ5OpLBPU+g3G/r5LSfS8w=
+github.com/lmittmann/tint v1.1.2/go.mod h1:HIS3gSy7qNwGCj+5oRjAutErFBl4BzdQP6cJZ0NfMwE=
 github.com/lucasb-eyer/go-colorful v1.2.0 h1:1nnpGOrhyZZuNyfu1QjKiUICQ74+3FNCN69Aj6K7nkY=
 github.com/lucasb-eyer/go-colorful v1.2.0/go.mod h1:R4dSotOR9KMtayYi1e77YzuveK+i7ruzyGqttikkLy0=
 github.com/lunixbochs/vtclean v1.0.0/go.mod h1:pHhQNgMf3btfWnGBVipUOjRYhoOsdGqdm/+2c2E2WMI=