It's more about distribution. So if you make some changes and keep it on your server you don't have to show the code to everyone, but if you release a plugin for third parties to download, then everyone who gets the download should be able to read, modify and distribute the plugin code under the same license. In this case the app is distributed to phones.
If your custom plugin distributes its output, no. Generally speaking, the output of a GPL'd program is not also subject to the GPL (quines notwithstanding.)
Indeed. I was hoping to elicit some clarification and left out some of my own: if JS, CSS & HTML are part of the GPL'd product, I'd agree that those parts qualify as being 'distributed' to users.
