PowerStroke 0.12.1 — Backup export fix

Back up your source, replace ContentView.swift, and build/run with the same Xcode project and app identifier. Do not delete the installed app. Existing profiles, photos and logs remain untouched.

The exporter now uses relative file names directly rather than deriving them from absolute iPad storage paths. An empty file collection produces an error instead of a successful vehicle-data backup message.

Test: confirm existing profile/logs are visible, open Backup, create a new backup, and confirm the saved-file count is greater than zero. Tap Save / share backup > Save to Files. Keep the original files. Restore is still not included.
