feat: add navigation links management and user registration

Features:
- Custom navigation links management in admin settings
- Multi-language support for navigation links (Chinese/English)
- Control link behavior (open in new window)
- Sort order and enable/disable toggle for nav links
- User registration system with validation
- Admin can enable/disable user registration
- Registration form with username, display name, email, password
- Link to registration from login page

Navigation Links:
- Admin interface to add/edit/delete custom nav links
- Support for both internal and external URLs
- Display links in header navigation bar
- Respects language preference

User Registration:
- Username validation (3-32 characters, alphanumeric + underscore/dash)
- Password validation (minimum 6 characters)
- Password confirmation matching
- Optional display name and email fields
- Can be toggled on/off by admin in site settings

Templates:
- Added navigation links settings page
- Added user registration page
- Updated login page with registration link
- Updated base layout to render custom nav links

Documentation:
- NAV_LINKS_FEATURE.md - Navigation links feature documentation
- REGISTRATION_FEATURE.md - User registration documentation
- IMPLEMENTATION_SUMMARY.md - Overall implementation summary

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

handlers/auth.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"net/http"
+	"strings"
 
 	"github.com/gin-contrib/sessions"
 	"github.com/gin-gonic/gin"
@@ -19,6 +20,11 @@ func LoginPage() gin.HandlerFunc {
 		if c.Query("error") == "1" {
 			data["Error"] = tr["login_error"]
 		}
+		// Check if registration is allowed from site settings
+		siteSetting, _ := c.Get("site_setting")
+		if s, ok := siteSetting.(*models.SiteSetting); ok && s != nil {
+			data["AllowRegistration"] = s.AllowRegistration
+		}
 		c.HTML(http.StatusOK, "login", data)
 	}
 }
@@ -73,3 +79,106 @@ func Logout() gin.HandlerFunc {
 		c.Redirect(http.StatusFound, "/")
 	}
 }
+
+// RegisterPage renders the registration form (only when registration is enabled).
+func RegisterPage(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		// Check if registration is allowed
+		var s models.SiteSetting
+		if err := db.First(&s, 1).Error; err != nil || !s.AllowRegistration {
+			c.Redirect(http.StatusFound, "/login")
+			return
+		}
+
+		tr := getTr(c)
+		data := DefaultData(c)
+		data["Title"] = tr["page_register"]
+
+		if errMsg := c.Query("error"); errMsg != "" {
+			data["Error"] = tr[errMsg]
+		}
+
+		c.HTML(http.StatusOK, "register", data)
+	}
+}
+
+// Register processes the registration form submission.
+func Register(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		// Check if registration is allowed
+		var s models.SiteSetting
+		if err := db.First(&s, 1).Error; err != nil || !s.AllowRegistration {
+			c.Redirect(http.StatusFound, "/login")
+			return
+		}
+
+		username := strings.TrimSpace(c.PostForm("username"))
+		password := c.PostForm("password")
+		confirmPassword := c.PostForm("confirm_password")
+		email := strings.TrimSpace(c.PostForm("email"))
+		displayName := strings.TrimSpace(c.PostForm("display_name"))
+
+		// Validate inputs
+		if username == "" || password == "" {
+			c.Redirect(http.StatusFound, "/register?error=register_required")
+			return
+		}
+
+		if len(username) < 3 || len(username) > 32 {
+			c.Redirect(http.StatusFound, "/register?error=register_username_length")
+			return
+		}
+
+		if len(password) < 6 {
+			c.Redirect(http.StatusFound, "/register?error=register_password_length")
+			return
+		}
+
+		if password != confirmPassword {
+			c.Redirect(http.StatusFound, "/register?error=register_password_mismatch")
+			return
+		}
+
+		// Check if username already exists
+		var existingUser models.User
+		if err := db.Where("username = ?", username).First(&existingUser).Error; err == nil {
+			c.Redirect(http.StatusFound, "/register?error=user_username_exists")
+			return
+		}
+
+		// Create new user
+		user := models.User{
+			Username:    username,
+			Email:       email,
+			DisplayName: displayName,
+			Role:        models.RoleAuthor,
+			Status:      models.StatusNormal,
+		}
+
+		if displayName == "" {
+			user.DisplayName = username
+		}
+
+		if err := user.SetPassword(password); err != nil {
+			c.Redirect(http.StatusFound, "/register?error=register_error")
+			return
+		}
+
+		if err := db.Create(&user).Error; err != nil {
+			c.Redirect(http.StatusFound, "/register?error=register_error")
+			return
+		}
+
+		// Auto-login after successful registration
+		session := sessions.Default(c)
+		session.Set("user_id", user.ID)
+		session.Set("username", user.Username)
+		if err := session.Save(); err != nil {
+			c.Redirect(http.StatusFound, "/login")
+			return
+		}
+
+		// Redirect to home page
+		c.Redirect(http.StatusFound, "/")
+	}
+}