Filename dialog for Save and for Rename

When you Stop (-> "Save"), it will prompt with the default name (date minus .gpx), suggesting the filename to begin with the date, but letting you edit the text field to any name you like. Same dialog if you click the (currently disabled, from #5) "Rename" icon in the Manage Tracks page. Try to save; if OK report OK, if not (permission, directory, duplicate, etc) report that and let the user try again.

Start at the Stop → Save flow and the Manage Tracks Rename entry point, then trace how each currently handles filenames and save results. Implement one editable filename dialog with the date-based default, and verify that successful saves report OK while permission, directory, duplicate, or other failures are reported and can be retried.

Written by the indexing model from the issue text.