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/layouts/base.html

@@ -40,6 +40,16 @@
                     </svg>
                     <span>{{index .Tr "home"}}</span>
                 </a>
+                {{range .NavLinks}}
+                <a href="{{.URL}}" {{if .OpenNew}}target="_blank" rel="noopener noreferrer"{{end}} class="text-gray-600 hover:text-blue-600 transition-colors text-sm flex items-center gap-1.5">
+                    <span>{{.Title $.Lang}}</span>
+                    {{if .OpenNew}}
+                    <svg class="w-3 h-3" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M10 6H6a2 2 0 00-2 2v10a2 2 0 002 2h10a2 2 0 002-2v-4M14 4h6m0 0v6m0-6L10 14"/>
+                    </svg>
+                    {{end}}
+                </a>
+                {{end}}
                 {{if .IsLoggedIn}}
                     <!-- Avatar Dropdown (click to toggle) -->
                     <div class="relative" id="avatarDropdown">