feat: add Settings FAB button, drag-and-drop positioning, and size/spacing controls

- Add Settings quick-access button to both main chat and DM views
- Make FAB container draggable via toggle button with position saved to localStorage
- Add button size and spacing sliders in Settings > Appearance tab
- Use CSS custom properties for dynamic FAB sizing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/index.html

@@ -124,6 +124,9 @@
     <button class="fab fab-contacts" data-bs-toggle="modal" data-bs-target="#contactsModal" title="Contact Management">
         <i class="bi bi-person-fill"></i>
     </button>
+    <button class="fab fab-settings" data-bs-toggle="modal" data-bs-target="#settingsModal" title="Settings">
+        <i class="bi bi-gear-fill"></i>
+    </button>
 </div>
 
 <!-- DM Modal (Full Screen) -->