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/pages/login.html

@@ -40,6 +40,13 @@
                 {{index .Tr "login_submit"}}
             </button>
         </form>
+
+        {{if .AllowRegistration}}
+        <div class="mt-4 text-center">
+            <span class="text-sm text-gray-600">{{index .Tr "login_no_account"}}</span>
+            <a href="/register" class="text-sm text-blue-600 hover:text-blue-700 font-medium ml-1">{{index .Tr "login_register_link"}}</a>
+        </div>
+        {{end}}
     </div>
 </section>
 {{template "footer" .}}