feat: show determinate backup progress

[AJV20]

Summary

• add a determinate backup progress fraction to BackupViewModel
• show a linear progress bar and percentage label when backup tools report percentages
• forward pymobiledevice3/idevicebackup2 stderr progress into the UI while preserving failure diagnostics
• verify successful backup commands actually produce complete backup metadata before reporting success
• track fallback idevicebackup2 backup/restore processes so cancellation can stop the correct process
• harden message exports: CSV escaping/formula neutralization, MBOX multi-attachment output, MBOX header sanitization, and specific Messages readiness copy when manifest selection fails
• handle decimal percentage output in the shared progress parser

Verification

• Scripts/regression/run.py (37 checks)
• swift build
• swift build -c release
• bash Scripts/build.sh
• launched .build/Phosphor.app and verified a visible window

Device detection note

Live probes on this Mac currently return no backup-capable iOS device over USB, network usbmux, libimobiledevice, or mobdev2 Bonjour, so no live backup/export run was possible in this pass.

[momenbasel]

Closing as superseded, not as a rejection - the work landed through two other PRs:

• The determinate progress bar is now in main via Enable Finder Wi-Fi sync from the device view #43 (BackupViewModel.progressFraction + the linear bar in BackupListView).
• The export hardening (CSV escaping, MBOX, metadata verification, fallback cancellation) is in main via fix: harden backup cancellation and export state #33, which was adopted as the single owner of that work.
  Merging this branch on top would have re-introduced the same code a second time and shipped a real crash: it adds @EnvironmentObject var deviceVM to SettingsView, but the Settings scene in PhosphorApp is Settings { SettingsView() } with no injection, so opening Settings would trap. If you still want the global backup banner as a standalone improvement, please reopen a small PR against current main with just that piece and the environment-object injection fixed.