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>

middleware/auth.go

@@ -131,6 +131,10 @@ func SetUserContext(db *gorm.DB) gin.HandlerFunc {
 		c.Set("site_home_subtitle", site.HomeSubtitle(string(lang)))
 		c.Set("site_footer_text", site.FooterText(string(lang)))
 
+		// --- Navigation links (from DB cache) ---
+		navLinks := models.GetNavLinks()
+		c.Set("nav_links", navLinks)
+
 		c.Next()
 	}
 }