feat(FN-707): hide project selector on mobile and add to overflow menu

- Hide ProjectSelector on mobile via hidden className and CSS media query
- Add project selector as menu item in mobile overflow menu
- Add CSS safety net (max-width: 767px) to ensure selector stays hidden
- Add comprehensive tests for mobile project selector visibility behavior

packages/dashboard/app/styles.css

@@ -4439,6 +4439,12 @@ body {
     padding: 12px 12px;
   }
 
+  /* Hide project selector and back button on mobile (belt-and-suspenders with conditional rendering) */
+  .header-project-selector,
+  .header-back-button {
+    display: none;
+  }
+
   /* Mobile header: collapsible search and overflow menu */
   .mobile-search-trigger {
     display: flex;