feat: add logout button to admin interface (Story 1.4)

Add logout button to admin dashboard and test to verify its presence.
This completes the missing acceptance criterion for Story 1.4:
"Logout option available from admin interface"

Changes:
- Add logout form with CSRF protection to dashboard header
- Add integration test to verify logout button is present
- All tests pass (24/24)

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

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

src/templates/admin/dashboard.html

@@ -6,6 +6,10 @@
 <article>
     <header>
         <h1>Admin Dashboard</h1>
+        <form method="POST" action="{{ url_for('admin.logout') }}" style="display: inline;">
+            <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
+            <button type="submit" class="secondary">Logout</button>
+        </form>
     </header>
 
     <p>Welcome to the Sneaky Klaus admin dashboard!</p>