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/models/application_platform.rb

@@ -0,0 +1,29 @@
+class ApplicationPlatform < PlatformAgent
+  def ios?
+    match? /iPhone|iPad/
+  end
+
+  def android?
+    match? /Android/
+  end
+
+  def mac?
+    match? /Macintosh/
+  end
+
+  def chrome?
+    user_agent.browser.match? /Chrome/
+  end
+
+  def safari?
+    user_agent.browser.match? /Safari/
+  end
+
+  def mobile?
+    ios? || android?
+  end
+
+  def desktop?
+    !mobile?
+  end
+end