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/helpers/hotkeys_helper.rb

@@ -0,0 +1,19 @@
+module HotkeysHelper
+  def hotkey_label(hotkey)
+    hotkey.split(",").first if hotkey
+    case hotkey
+    when Array
+      hotkey.map { |key|
+        if key == "ctrl" && platform.mac?
+          "⌘"
+        elsif key == "enter"
+          platform.mac? ? "return" : "enter"
+        else
+          key
+        end
+      }.join("+")
+    else
+      hotkey.split(",").first if hotkey
+    end
+  end
+end