feat(saves): back up a save directly to a USB drive on the PS5

[Twice6804]

Pull Request

Description

Adds a "Save to USB storage" action to the Saves screen that backs a save up
directly to a USB/extended-storage drive plugged into the PS5 itself,
without round-tripping through a host PC. The destination base path is
configurable in Settings (default /mnt/usb0/savedata); each backup lands
at <savePath>/<title_id>/<timestamp>/<title_id>.zip.

Addresses the first half of #144 ("Save a complete backup of all save games
in one click... in a folder on the USB drive with timestamps folders").

Type of Change

• New feature (non-breaking change which adds functionality)

Changes Made

• New savePath setting (Settings screen + localStorage + ~/.ps5upload/settings.json mirror), default /mnt/usb0/savedata.
• New per-row "Save to USB" button and a header "Back up all to USB" bulk button on the Saves screen.
• The backup flow reuses the existing download → finalize → zip pipeline, then uploads the zip back onto the PS5 via the existing transfer_file single-file upload — no new wire protocol, no payload/engine changes.
• Pre-flights the USB mount (writable, not a placeholder) via the existing checkDestinationFreeSpace helper and re-checks for a stale host before uploading, mirroring the Restore flow's existing safety guards.
• New backupTimestamp helper (YYYY-MM-DD_HHMMSS, local time) and saveSettings store, both unit-tested.

Documentation Updates

Did you update the documentation? [ ] Yes [x] No

The existing Backup/Restore save feature isn't documented in README/FAQ either; this follows the same precedent — the feature is self-explanatory from the Saves/Settings UI.

Testing

• npm run validate (lint, typecheck, full Vitest suite — 754 passing, i18n coverage gate, Vite production build all green)
• npm run coverage
• Built PS5 payload successfully (no payload changes in this PR)
• Cross-platform CI target checks passed
• Tested on real PS5 hardware
• Tested desktop app
• Verified all example commands in docs work
• Tested edge cases and error handling

Note: the Rust cargo check for the desktop Tauri shell could not be run in my sandbox (missing system GTK/WebKit dev packages, e.g. libwebkit2gtk-4.1-dev / libgtk-3-dev) — a pre-existing environment limitation, not something this PR's code touches (zero Rust files changed). CI will run the full gate.

I also don't have PS5 hardware to test against in this environment — would appreciate a hardware smoke test (plug a USB stick into the console, run a per-row and a bulk backup, confirm the zip lands at the expected path and round-trips through Restore) before merge.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation to reflect my changes
• My changes generate no new warnings
• I have tested my changes on PS5 hardware (or simulated environment)
• All example code in documentation has been tested and works

Additional Notes

Part 2 of #144 (convert game folders to/from exfat/ufs via UFS2Tool, and zip/7z on-console for non-save game backups) is out of scope for this PR.

[Twice6804]

@phantomptr quick PR to backup save games to usb drive, adresses the first part of #144