desktop: reminders reach you when the window isn't in front

A Rust worker reads due reminders from the local store every 15 s and
announces each occurrence once: always to the main window as a toast, and
as a system notification (tauri-plugin-notification) when that window
isn't focused. The page no longer polls on the desktop; its Notification
went nowhere in WebKitGTK and its timer stopped with the window. The
Reminders page says what each surface actually does.

Core gains store::due_reminders, compared by instant, not by string.

Refs #5171

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

desktop/src-tauri/Cargo.toml

@@ -69,3 +69,9 @@ tauri-plugin-updater = "2"
 # concept of a global shortcut, and its half of this feature is a share-sheet
 # intent filter instead.
 tauri-plugin-global-shortcut = "2"
+
+# System notifications for due reminders (#5171), raised from the reminder worker in
+# Rust. The page's own `Notification` goes nowhere in WebKitGTK, and the page's
+# timer only runs while its window does. Used from Rust only, so no capability
+# grants it to the webview.
+tauri-plugin-notification = "2"