Add native notification scheduler for workday reminders

[BrainerVirus]

Summary

Build a Rust-backed service that schedules OS-native reminders before the workday ends.

Scope

• Add the Tauri notification plugin and wire it into the desktop app.
• Schedule reminders relative to the user's work schedule, not a fixed clock.
• Recompute the next reminder when schedule or preferences change.
• Keep the runtime lightweight: one deadline-based timer, no frequent polling.

Platform notes

• macOS may require notification permission.
• Linux should follow the desktop notification daemon.
• Windows notifications work best in installed apps.

Acceptance criteria

• Timely can send a native notification from the desktop runtime.
• The scheduler works from tray/background mode while the app is running.
• Reminder timing is derived from the user's shift end and remaining work time.
• The implementation does not use fake fallback notifications in runtime code.