## Problem

On multi-monitor setups, notification toast popups appear on every connected output simultaneously. The same notification card is duplicated on each screen.

## Root cause

In `omarchy.notifications` `Service.qml`, the toast popup window is created for all outputs via:

```qml
Variants {
    model: Quickshell.screens  // one PanelWindow per output
    PanelWindow {
        screen: modelData
        Repeater { model: popupModel }  // shared model, no per-screen filter
    }
}
```

Every PanelWindow renders from the same shared `popupModel`, so each monitor shows identical toasts.

## Suggested fix

Add a configurable primary output (or default to the focused/hyprland primary monitor), and only render toasts on that screen. Could be:

1. A `shell.toml` option like `notifications.output = DP-1`
2. Or default to `Hyprland.primaryMonitor` / first screen in `Quickshell.screens`

The change would be \~5 lines: skip creating the PanelWindow for non-primary outputs, or filter the Repeater model per screen.

## Use case

Dual-monitor users typically want notifications consolidated on one display rather than duplicated across all screens.