Clean up redundant authentication checks in dashboard template

Removed redundant current_user.is_authenticated checks in the else block
of the navigation menu. The else block only executes for authenticated
users per app.py logic (line 278 vs 293), so the nested checks were
dead code that created confusion.

Changes:
- Removed defensive checks for unauthenticated users in authenticated block
- Added clarifying comment about when else block executes
- Simplified template logic for better maintainability
- Removed dead code paths (Anonymous User label, ? avatar)

Addresses concerns raised in Issue #2 about confusing template logic.

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

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

templates/dashboard.html

@@ -27,24 +27,17 @@
                     <a href="{{ url_for('register') }}" class="register-btn">📝 Sign Up</a>
                 </div>
             {% else %}
+                {# This block only executes for authenticated users (per app.py line 278) #}
                 <div class="user-menu">
                     <div class="user-info">
                         <div class="user-avatar">
-                            {% if current_user.is_authenticated %}
                             {% if current_user.profile_picture_url %}
                                 <img src="{{ current_user.profile_picture_url }}" alt="Avatar">
                             {% else %}
                                 <div class="avatar-placeholder">{{ current_user.username[:2].upper() }}</div>
                             {% endif %}
-                            {% else %}
-                                <div class="avatar-placeholder">?</div>
-                            {% endif %}
                         </div>
-                        {% if current_user.is_authenticated %}
                         <span class="username">{{ current_user.username }}</span>
-                        {% else %}
-                            <span class="username">Anonymous User</span>
-                        {% endif %}
                     </div>
                     <div class="user-dropdown">
                         <a href="{{ url_for('settings') }}" class="dropdown-item">⚙️ Settings</a>