Opt-in desktop notifications for sync events

Goal

Today the only way to know that something happened (a peer connected, a conflict was created, a directory was added) is to have the GUI open. Add OS-level desktop notifications so important events surface even when the browser tab is closed.

Requirements

• New app/src/infra/notifications/ module using a small cross-platform crate such as notify-rust. Behind the existing port pattern if there's an obvious home, otherwise an in-place adapter is fine for this iteration.
• Subscribe the notifier to the same ServerEvent broadcast that drives SSE (AppState::sse_sender()), so we don't grow a second event bus.
• Emit notifications for at least:
  • Peer connected / disconnected
  • Conflict created (depends on / aligns with the conflicts GUI issue)
  • Sync directory added / removed
• Opt-in via config.toml. Default is off. Per-event-class toggles would be nice but a single notifications.enabled = true is acceptable for this iteration.
• Failures to display a notification must not crash or block the synchronizer — log at warn and move on.

Out of scope

• A system tray / menu-bar icon (separate, larger issue)
• Notification actions (''click to open GUI'') — nice to have, but optional for this iteration
• Aggregating notification storms (revisit if it turns out to be annoying)

Acceptance criteria

• With notifications.enabled = true, connecting a peer on another device fires a desktop notification
• With notifications disabled (default), no notifications fire even when events occur
• The notifier never blocks ServerEvent propagation to the GUI
• Linux (libnotify) verified; macOS / Windows ''best-effort'' is acceptable for now if the crate covers them