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/settings.go

@@ -100,6 +100,7 @@ func SiteSettingsSave(db *gorm.DB, storagePath string) gin.HandlerFunc {
 		s.HomeSubtitleEn = strings.TrimSpace(c.PostForm("home_subtitle_en"))
 		s.FooterTextZh = strings.TrimSpace(c.PostForm("footer_text_zh"))
 		s.FooterTextEn = strings.TrimSpace(c.PostForm("footer_text_en"))
+		s.AllowRegistration = c.PostForm("allow_registration") == "1"
 		s.UpdatedBy = userIDFromSession(c)
 
 		// Favicon upload (optional). A favicon_url form field takes precedence over an
@@ -427,3 +428,104 @@ func CommentSettingsSave(db *gorm.DB) gin.HandlerFunc {
 		c.Redirect(http.StatusFound, "/admin/settings/comments?saved=1")
 	}
 }
+
+// ---------------- Navigation Links settings ----------------
+
+// NavLinksSettingsPage renders the navigation links management page.
+func NavLinksSettingsPage(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		tr := getTr(c)
+		var links []models.NavLink
+		db.Order("sort asc, id asc").Find(&links)
+
+		data := DefaultData(c)
+		data["Title"] = tr["settings_navlinks_title"]
+		data["NavLinks"] = links
+		if msg := c.Query("saved"); msg == "1" {
+			data["Success"] = tr["settings_saved"]
+		}
+		c.HTML(http.StatusOK, "settings_navlinks", data)
+	}
+}
+
+// NavLinksSettingsSave dispatches navigation link actions.
+func NavLinksSettingsSave(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		switch c.PostForm("action") {
+		case "add":
+			addNavLink(db, c)
+		case "toggle":
+			toggleNavLink(db, c)
+		case "edit":
+			editNavLink(db, c)
+		case "delete":
+			deleteNavLink(db, c)
+		}
+		models.RefreshConfigCache(db)
+		c.Redirect(http.StatusFound, "/admin/settings/navlinks?saved=1")
+	}
+}
+
+func addNavLink(db *gorm.DB, c *gin.Context) {
+	titleZh := strings.TrimSpace(c.PostForm("title_zh"))
+	titleEn := strings.TrimSpace(c.PostForm("title_en"))
+	url := strings.TrimSpace(c.PostForm("url"))
+
+	if url == "" || (titleZh == "" && titleEn == "") {
+		return
+	}
+
+	sort, _ := strconv.Atoi(c.PostForm("sort"))
+
+	link := models.NavLink{
+		TitleZh:  titleZh,
+		TitleEn:  titleEn,
+		URL:      url,
+		OpenNew:  c.PostForm("open_new") == "1",
+		Enabled:  c.PostForm("enabled") != "0",
+		Sort:     sort,
+		UpdatedBy: userIDFromSession(c),
+	}
+	db.Create(&link)
+}
+
+func toggleNavLink(db *gorm.DB, c *gin.Context) {
+	id, _ := strconv.Atoi(c.PostForm("id"))
+	var link models.NavLink
+	if db.First(&link, id).Error != nil {
+		return
+	}
+	link.Enabled = !link.Enabled
+	link.UpdatedBy = userIDFromSession(c)
+	db.Save(&link)
+}
+
+func editNavLink(db *gorm.DB, c *gin.Context) {
+	id, _ := strconv.Atoi(c.PostForm("id"))
+	var link models.NavLink
+	if db.First(&link, id).Error != nil {
+		return
+	}
+
+	titleZh := strings.TrimSpace(c.PostForm("title_zh"))
+	titleEn := strings.TrimSpace(c.PostForm("title_en"))
+	url := strings.TrimSpace(c.PostForm("url"))
+
+	if url == "" || (titleZh == "" && titleEn == "") {
+		return
+	}
+
+	link.TitleZh = titleZh
+	link.TitleEn = titleEn
+	link.URL = url
+	link.OpenNew = c.PostForm("open_new") == "1"
+	link.Sort, _ = strconv.Atoi(c.PostForm("sort"))
+	link.UpdatedBy = userIDFromSession(c)
+	db.Save(&link)
+}
+
+func deleteNavLink(db *gorm.DB, c *gin.Context) {
+	id, _ := strconv.Atoi(c.PostForm("id"))
+	db.Delete(&models.NavLink{}, id)
+}
+