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>

templates/admin/settings_download.html

@@ -6,6 +6,7 @@
 
     <div class="flex gap-2 mb-8">
         <a href="/admin/settings/site" class="px-4 py-2 rounded-lg text-sm font-medium bg-gray-200 text-gray-700 hover:bg-gray-300 transition-colors">{{index .Tr "settings_site_title"}}</a>
+        <a href="/admin/settings/navlinks" class="px-4 py-2 rounded-lg text-sm font-medium bg-gray-200 text-gray-700 hover:bg-gray-300 transition-colors">{{index .Tr "settings_navlinks_title"}}</a>
         <a href="/admin/settings/upload" class="px-4 py-2 rounded-lg text-sm font-medium bg-gray-200 text-gray-700 hover:bg-gray-300 transition-colors">{{index .Tr "settings_upload_title"}}</a>
         <a href="/admin/settings/download" class="px-4 py-2 rounded-lg text-sm font-medium bg-blue-600 text-white">{{index .Tr "settings_download_title"}}</a>
     </div>