Add global registration disable feature

Implement configurable registration control to allow administrators to disable new user signups.

Backend changes:
- Add fitpub.registration.enabled configuration property (defaults to true)
- Update AuthController to check registration status and return 403 Forbidden when disabled
- Create GET /api/auth/registration-status endpoint to expose registration status to frontend
- Add RegistrationStatusResponse DTO

Configuration changes:
- Add REGISTRATION_ENABLED environment variable to application.yml
- Add REGISTRATION_ENABLED to Dockerfile with default value of true
- Update .env.example with REGISTRATION_ENABLED documentation

Frontend changes:
- Update registration page to check status and hide form when disabled
- Add checkRegistrationStatus() to auth.js to dynamically hide registration links
- Display user-friendly message when registration is disabled

To disable registration, set environment variable: REGISTRATION_ENABLED=false

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/main/resources/static/js/auth.js

@@ -173,6 +173,9 @@ const FitPubAuth = {
         // Update navigation UI based on auth status
         this.updateNavigationUI();
 
+        // Check registration status and update UI
+        this.checkRegistrationStatus();
+
         // Check authentication status on page load
         this.checkAuthStatus();
 
@@ -180,6 +183,30 @@ const FitPubAuth = {
         this.setupExpirationWarning();
     },
 
+    /**
+     * Check if registration is enabled and update navigation UI
+     */
+    checkRegistrationStatus: async function() {
+        try {
+            const response = await fetch('/api/auth/registration-status');
+            const data = await response.json();
+
+            if (!data.enabled) {
+                // Hide registration link in navigation
+                const registerLinks = document.querySelectorAll('a[href="/register"]');
+                registerLinks.forEach(link => {
+                    const parent = link.parentElement;
+                    if (parent && parent.tagName === 'LI') {
+                        parent.style.display = 'none';
+                    }
+                });
+            }
+        } catch (error) {
+            console.error('Error checking registration status:', error);
+            // Continue without hiding registration link if check fails
+        }
+    },
+
     /**
      * Update navigation UI based on authentication status
      */