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>

models/config_cache.go

@@ -17,6 +17,7 @@ var configCache = struct {
 	comment  *CommentConfig
 	types    []UploadFileType
 	baseURLs []DownloadBaseURL
+	navLinks []NavLink
 }{
 	site:   &SiteSetting{},
 	upload: &UploadConfig{Enabled: true, DefaultMaxSize: DefaultUploadMaxSize, StorageDir: "attachments"},
@@ -58,6 +59,11 @@ func LoadConfigCache(db *gorm.DB) {
 	if err := db.Order("is_default desc, priority asc, id asc").Find(&b).Error; err == nil {
 		configCache.baseURLs = b
 	}
+
+	var n []NavLink
+	if err := db.Where("enabled = ?", true).Order("sort asc, id asc").Find(&n).Error; err == nil {
+		configCache.navLinks = n
+	}
 }
 
 // RefreshConfigCache reloads all cached platform configuration. Admin handlers
@@ -118,3 +124,10 @@ func DefaultDownloadBaseURL() string {
 	}
 	return ""
 }
+
+// GetNavLinks returns the cached list of enabled navigation links, sorted by sort order.
+func GetNavLinks() []NavLink {
+	configCache.mu.RLock()
+	defer configCache.mu.RUnlock()
+	return configCache.navLinks
+}