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>

version.go

@@ -1,14 +1,13 @@
 package pluginctl
 
 import (
-	"fmt"
 	"runtime/debug"
 )
 
 // RunVersionCommand implements the 'version' command functionality.
 func RunVersionCommand(args []string) error {
 	version := GetVersion()
-	fmt.Printf("pluginctl version %s\n", version)
+	Logger.Info("pluginctl version", "version", version)
 
 	return nil
 }