maiden should check for on device file modification before run or save

[ngwese]

https://llllllll.co/t/norns-maiden/14052/137
https://llllllll.co/t/norns-maiden/14052/138

what might make the most sense is for maiden to check to see; on run or save, if the file back on norns has changed - if it has then maiden could present a warning that work could be overwritten and offer to continue or to save the current file with a new name.

given the lack of a real time clock on device we can't use timestamps to check for change but simply doing a content hash would work and not be too costly. on run/save the editor could compare the hash of the in browser buffer with a hash obtained from the backend.