### Environment
- OS: Omarchy 4.0.4-1 (Linux 7.2.5-3-omarchy)
- Hyprland: 0.56.2-2
- Quickshell: 0.3.1 (Qt 6.11.2)
- Hardware: Single monitor (DP-1), NVIDIA GP108 [GeForce GT 1030]

### Description
When the screen is locked and idle blanking triggers, `omarchy-brightness-display off` executes `hyprctl dispatch 'hl.dsp.dpms({ action = "disable" })'`. Disabling DPMS on the only connected display causes Hyprland to remove all Wayland outputs.

Qt Wayland creates a placeholder screen (`QScreen(name="")`). Quickshell's bar delegates (`Variants { model: Quickshell.screens }` in `plugins/bar/Bar.qml` and `plugins/background/Background.qml`) instantiate `PanelWindow` / `WlrLayershell` surfaces against the placeholder screen. On expose, `QWaylandGLContext::makeCurrent` fails with `eglError: 0x3003` (`EGL_BAD_ALLOC`), causing Qt Quick RHI initialization to fail and aborting the process via `qFatal("Failed to initialize graphics backend for OpenGL.")`.

`omarchy-launch-shell` attempts 6 relaunches while the monitor is still powered down, each aborting immediately on startup until the supervisor gives up.

### Steps to Reproduce
1. On a single-monitor setup, lock the session.
2. Wait for `idleBlankTimer` (5s) to trigger screen blanking (`omarchy-brightness-display off` -> DPMS disable).
3. The display is disabled, Hyprland removes all outputs, and Qt creates a placeholder screen.
4. Quickshell crashes with `SIGABRT` due to `FATAL: Failed to initialize graphics backend for OpenGL.` (`EGL_BAD_ALLOC` on placeholder screen).
5. `omarchy-launch-shell` relaunches `quickshell` up to 6 times until exceeding the relaunch budget.

### Expected Behavior
`quickshell` should not attempt to bind layer-shell surfaces to placeholder screens lacking a valid backing Wayland output (similar to how `plugins/lock/Service.qml` guards against placeholder screens: `DEBUG: Not creating lock surface for screen ... as it is not backed by a valid wayland output`), or handle zero-output states gracefully without aborting.

### Stack Trace
```
#0  __pthread_kill_implementation () at pthread_kill.c:44
#1  raise () from libc.so.6
#2  abort () from libc.so.6
#3  qAbort () from libQt6Core.so.6
#4  QMessageLogger::fatal () from libQt6Core.so.6
#5  QQuickWindow::event () from libQt6Quick.so.6
#6  QCoreApplication::notifyInternal2 () from libQt6Core.so.6
#7  QGuiApplicationPrivate::processExposeEvent () from libQt6Gui.so.6
#8  QtWaylandClient::QWaylandWindow::sendExposeEvent () from libQt6WaylandClient.so.6
#9  QtWaylandClient::QWaylandWindow::updateExposure () from libQt6WaylandClient.so.6
```

> Filed by gemini-3.8-flash via Oh My Pi.