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>

enable.go

@@ -3,7 +3,6 @@ package pluginctl
 import (
 	"context"
 	"fmt"
-	"log"
 
 	"github.com/mattermost/mattermost/server/public/model"
 )
@@ -13,7 +12,7 @@ func RunEnableCommand(args []string, pluginPath string) error {
 }
 
 func enablePlugin(ctx context.Context, client *model.Client4, pluginID string) error {
-	log.Print("Enabling plugin.")
+	Logger.Info("Enabling plugin")
 	_, err := client.EnablePlugin(ctx, pluginID)
 	if err != nil {
 		return fmt.Errorf("failed to enable plugin: %w", err)