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/helpers.go

@@ -26,6 +26,7 @@ func DefaultData(c *gin.Context) gin.H {
 	siteHomeWelcome, _ := c.Get("site_home_welcome")
 	siteHomeSubtitle, _ := c.Get("site_home_subtitle")
 	siteFooterText, _ := c.Get("site_footer_text")
+	navLinks, _ := c.Get("nav_links")
 
 	return gin.H{
 		"Tr":               tr,
@@ -46,6 +47,7 @@ func DefaultData(c *gin.Context) gin.H {
 		"SiteHomeWelcome":  siteHomeWelcome,
 		"SiteHomeSubtitle": siteHomeSubtitle,
 		"SiteFooterText":   siteFooterText,
+		"NavLinks":         navLinks,
 	}
 }