Proactive desktop notification for the deletion guard

[gnacho]

Summary

Closes #203. Ports the proactive desktop-notification flow from PyNextCloud-Sync 0.1.32 to the deletion guard.

When the guard pauses synchronization on a mass local deletion, the app now raises a critical desktop notification (urgency Critical) with a Review Now button that opens the deletion review directly, even when the app runs only in the tray. Previously the state was only reflected in the tray/UI and the user was never proactively notified.

Changes

• core/notifications.rs: add DesktopNotifier::send_delete_review(summary, body, on_action) — raises a critical notify-rust notification with a "Review Now" action and fires on_action on a worker thread.
• core/scheduler.rs: add a delete_review_cb hook (set_on_delete_review) fired with the alert on raise and cleared on approve/restore/stop.
• ui/main_window.rs:
  • install_delete_review_handler: wires the hook per folder, builds the copy, and routes "Review Now" back to the review through an mpsc channel + a main-loop poller (a non-Send window Weak cannot cross the notifier thread).
  • Present the review dialog once the main window is mapped, so it works when the app is only in the tray (window hidden).
• main.rs: wire the handler after the window is built.
• i18n: new "Review Now" and "Synchronization was paused before {count} files could be deleted from Nextcloud." strings (ES).

Validation

• 708 tests passed + 1 ignored (known flaky login_flow), clippy --all-targets -D warnings and cargo fmt clean.
• End-to-end: a folder with removed files raised the critical notification; "Review Now" opened the deletion review (verified in the tray/UI).
• Version bumped to 0.2.18 (CHANGELOG, version.json, metainfo, READMEs, landing).