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.