integrate: FG Scan documentation into help system

- Add fg_scan.md to documentation routes mapping
- Add FG Scan link to help viewer navigation menu
- Add floating help button to fg_scan.html page
- Documentation now accessible from both:
  * Floating help button (📖) on fg_scan page
  * Help navigation menu in documentation viewer

py_app/app/templates/fg_scan.html

@@ -839,6 +839,13 @@ window.onclick = function(event) {
 
 {% endblock %}
 {% block content %}
+<!-- Floating Help Button -->
+<div class="floating-help-btn">
+    <a href="{{ url_for('main.help', page='fg_scan') }}" target="_blank" title="FG Scan Help">
+        📖
+    </a>
+</div>
+
 <div class="scan-container">
     <!-- Input Form Card -->
     <div class="card scan-form-card">