Detect platform and label the correct hotkey shortcut

Yes, Platform Agent is overkill for just this but there isn't a one-size JS solution for updating both input placeholders and other HTML text and we're likely to need it later for other things like displaying platform-specific PWA prompts.

app/controllers/concerns/set_platform.rb

@@ -0,0 +1,12 @@
+module SetPlatform
+  extend ActiveSupport::Concern
+
+  included do
+    helper_method :platform
+  end
+
+  private
+    def platform
+      @platform ||= ApplicationPlatform.new(request.user_agent)
+    end
+end