feat: Add validation method to CLI configuration with comprehensive field checks

- Add Validate method to CLIConfig struct returning slice of validation errors
- Add API endpoint validation (required, valid URL format, http/https scheme check)
- Add API token validation (empty warning, minimum 10 character length check)
- Add profile validation against allowed values (default, dev, staging, prod)
- Add theme validation against allowed values (default, light, dark, high-contrast)
- Add imports for sl

apps/cli/internal/types/config.go

@@ -1,5 +1,12 @@
 package types
 
+import (
+	"slices"
+	"fmt"
+	"net/url"
+	"strings"
+)
+
 type CLIConfig struct {
 	APIEndpoint string
 	APIToken    string
@@ -8,3 +15,53 @@ type CLIConfig struct {
 	NoColor     bool
 	Theme       string
 }
+
+// Validate checks the configuration for errors and returns a list of validation errors
+func (c *CLIConfig) Validate() []error {
+	var errors []error
+
+	// Validate API endpoint
+	if c.APIEndpoint == "" {
+		errors = append(errors, fmt.Errorf("API endpoint is required"))
+	} else {
+		if _, err := url.Parse(c.APIEndpoint); err != nil {
+			errors = append(errors, fmt.Errorf("invalid API endpoint URL: %w", err))
+		}
+		if !strings.HasPrefix(c.APIEndpoint, "http://") && !strings.HasPrefix(c.APIEndpoint, "https://") {
+			errors = append(errors, fmt.Errorf("API endpoint must start with http:// or https://"))
+		}
+	}
+
+	// Validate API token (optional but warn if empty)
+	if c.APIToken == "" {
+		errors = append(errors, fmt.Errorf("API token is empty (some operations may fail)"))
+	} else if len(c.APIToken) < 10 {
+		errors = append(errors, fmt.Errorf("API token appears too short (expected at least 10 characters)"))
+	}
+
+	// Validate profile
+	validProfiles := []string{"default", "dev", "staging", "prod"}
+	if c.Profile != "" {
+		valid := slices.Contains(validProfiles, c.Profile)
+		if !valid {
+			errors = append(errors, fmt.Errorf("invalid profile '%s', must be one of: %v", c.Profile, validProfiles))
+		}
+	}
+
+	// Validate theme
+	validThemes := []string{"default", "light", "dark", "high-contrast"}
+	if c.Theme != "" {
+		valid := false
+		for _, t := range validThemes {
+			if c.Theme == t {
+				valid = true
+				break
+			}
+		}
+		if !valid {
+			errors = append(errors, fmt.Errorf("invalid theme '%s', must be one of: %v", c.Theme, validThemes))
+		}
+	}
+
+	return errors
+}