A bar cloned with `omarchy plugin clone omarchy.bar` never loads. The bar
disappears entirely and does not fall back to the built-in bar.

This makes the documented customization path for the bar unusable — the same one
`plugins.md` and `omarchy plugin clone --help` point users at.

### Steps to reproduce

```bash
omarchy plugin clone omarchy.bar   # clones and switches to <user>.bar
omarchy restart shell
```

No edits to the clone are needed. The bar surface is gone
(`hyprctl layers` shows no `omarchy-bar` layer), and the shell logs:

```
WARN scene: file:///home/.../plugins/<user>.bar/Bar.qml[15:3]: Required property omarchyPath was not initialized
WARN scene: file:///home/.../plugins/<user>.bar/Bar.qml[17:3]: Required property barWidgetRegistry was not initialized
WARN scene: file:///home/.../plugins/<user>.bar/Bar.qml[22:3]: Required property barConfig was not initialized
WARN scene: @shell.qml[256:-1]: ReferenceError: errorString is not defined
```

**Expected:** the cloned bar loads and behaves identically to the built-in one,
since its contents are a byte-for-byte copy.

### Cause

Two separate issues, both in `shell/shell.qml`.

**1. Plugin bars receive their required properties after instantiation.**

The built-in bar is built from an inline `Component` that sets them
declaratively, which satisfies `required` (shell.qml:225-234):

```qml
Component {
  id: defaultBarComponent
  Bar {
    omarchyPath: shell.omarchyPath
    barWidgetRegistry: shell.barWidgetRegistry
    barConfig: shell.barConfig
    ...
  }
}
```

A plugin bar instead loads by URL and is configured from `onLoaded`
(shell.qml:246-252):

```qml
Loader {
  id: pluginBarLoader
  source: shell.activeBarId !== shell.defaultBarId ? shell.activeBarSourceUrl : ""
  asynchronous: true
  onLoaded: shell.configureBar(item, shell.activeBarManifest)
  ...
}
```

`configureBar()` (shell.qml:214-223) assigns `omarchyPath`,
`barWidgetRegistry` and `barConfig` — but QML enforces `required property` at
construction, so the object is never created and `onLoaded` never runs. Since
`shell/plugins/bar/Bar.qml` declares all three as `required` (lines 15, 17, 22),
any faithful clone of it hits this. Any third-party bar copying the built-in
bar's structure would too.

A `Loader` can supply initial properties, which would fix this without changing
the packaged `Bar.qml`:

```qml
pluginBarLoader.setSource(url, {
  omarchyPath: shell.omarchyPath,
  barWidgetRegistry: shell.barWidgetRegistry,
  barConfig: shell.barConfig,
  shell: shell,
  manifest: shell.activeBarManifest
})
```

**2. The fallback to the built-in bar throws before it can run.**

`Loader` has no `errorString` in Qt 6 QML — it is a member of `QQmlComponent`,
which is why the `comp.errorString()` calls at shell.qml:297, 801 and 804 are
fine. The bare call at shell.qml:256 is not:

```qml
var detail = errorString && errorString() ? errorString() : ""
```

The `errorString &&` guard does not help: referencing an undefined *identifier*
raises `ReferenceError` rather than evaluating to `undefined`. The exception
escapes before `shell.failedBarId = shell.activeBarId` runs, so the designed
fallback to `omarchy.bar` never happens and the user is left with no bar at all
rather than a degraded one.

shell.qml:645 has the same bare call, with a comment stating that
`Loader.errorString()` reflects the source-load failure, so the same fix likely
applies there. Line 646 already falls back to `sourceComponent.errorString()`,
which is the correct API.

Worth noting that issue 2 is what turns issue 1 into a blank desktop. With the
fallback working, a broken bar plugin would degrade to the built-in bar instead.

### Workaround

Change the three root-level `required property` declarations in the clone's
`Bar.qml` to plain properties with defaults:

```qml
property string omarchyPath: ""
property var barWidgetRegistry: null
property var barConfig: null
```

The delegate-scoped `required property var modelData`/`entry` declarations
further down the file are fine and should be left alone. `configureBar()`
assigns the real values immediately on load and everything downstream reads them
through bindings, so nothing else needs changing.

### System details

| | |
|---|---|
| Omarchy | 4.0.0-1 (omarchy 4.0.0-1, omarchy-settings 4.0.0-1) |
| CPU | Intel Core i5-7Y57 |
| GPU | Intel HD Graphics 615 (Kaby Lake-Y GT2) |
| Kernel | 7.1.8-arch1-3 |
| qt6-base | 6.11.1-1 |
| quickshell-git | 0.3.0.r20.g28771c7-1 |
| hyprland | 0.56.2-1 |

Upgraded from 3.8.5 via `omarchy-upgrade-to-quattro`. Reproduced on a clean
clone with no modifications; the hardware looks incidental, since both causes are
in QML control flow.

No screenshot attached — the visual symptom is simply an absent bar, and the log
plus the two code paths above characterise it more precisely than an image of an
empty desktop would.