Fix 500 error: Change register route to signup

Fixed BuildError caused by incorrect endpoint name in anonymous mode.
The route is called 'signup' not 'register' in app.py line 878.

Error was:
werkzeug.routing.exceptions.BuildError: Could not build url for endpoint 'register'

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

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

templates/dashboard.html

@@ -24,7 +24,7 @@
             {% if anonymous %}
                 <div class="anonymous-actions">
                     <a href="{{ url_for('login') }}" class="login-btn">🔑 Login</a>
-                    <a href="{{ url_for('register') }}" class="register-btn">📝 Sign Up</a>
+                    <a href="{{ url_for('signup') }}" class="register-btn">📝 Sign Up</a>
                 </div>
             {% else %}
                 {# This block only executes for authenticated users (per app.py line 278) #}