Use x_user_agent cookie for platform detection in Hotwire Native

Turbo's offline/service worker sets an x_user_agent cookie with the
original browser user agent. This ensures platform detection works
correctly for Hotwire Native apps where service worker requests may
not carry the overridden user agent header.

test/controllers/concerns/set_platform_test.rb

@@ -0,0 +1,21 @@
+require "test_helper"
+
+class SetPlatformTest < ActionDispatch::IntegrationTest
+  DESKTOP_UA = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36"
+  NATIVE_IOS_UA = "Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Hotwire Native iOS/1.0"
+
+  test "uses the request user agent by default" do
+    sign_in_as :david
+
+    get board_path(boards(:writebook)), headers: { "User-Agent" => DESKTOP_UA }
+    assert_match 'data-platform="desktop web"', response.body
+  end
+
+  test "prefers x_user_agent cookie over request user agent" do
+    sign_in_as :david
+
+    cookies[:x_user_agent] = NATIVE_IOS_UA
+    get board_path(boards(:writebook)), headers: { "User-Agent" => DESKTOP_UA }
+    assert_match 'data-platform="native ios"', response.body
+  end
+end