fix(form): preserve 480px user bar boundary

Keep the mobile layout below 480px and lock both sides of the breakpoint with browser coverage.

tests/test_generate_form.py

@@ -432,3 +432,15 @@ class TestGenerateFormGooglePlaywright:
             })"""
         )
         assert dimensions["scrollWidth"] == dimensions["clientWidth"]
+
+    def test_user_bar_wraps_below_shared_mobile_breakpoint(
+        self, signed_in_admin_page
+    ):
+        page = signed_in_admin_page
+        page.set_viewport_size({"width": 479, "height": 800})
+        assert (
+            page.locator(".user-bar").evaluate(
+                "(element) => getComputedStyle(element).flexWrap"
+            )
+            == "wrap"
+        )