feat(02-01): add Files navigation to all pages

- Added Files menu link to dashboard, settings, and calibration templates
- Follows existing nav structure with nav-btn class and Bootstrap Icons
- Enables navigation to /files route for file management page

Part of 02-01 file management plan

src/templates/settings.html

@@ -175,6 +175,9 @@
         <a href="/calibration" class="nav-btn">
             <i class="bi bi-sliders"></i> Calibration
         </a>
+        <a href="/files" class="nav-btn">
+            <i class="bi bi-folder"></i> Files
+        </a>
     </nav>
     
     <div class="container-fluid p-0">