### What happened

`omarchy plugin clone omarchy.bar` produces a bar that never renders. There is no
`omarchy-bar` layer in `hyprctl layers`, and the screen has no bar at all — the
shell does not fall back to the built-in one either.

This is not caused by editing the clone: a byte-identical copy of the packaged
`shell/plugins/bar/Bar.qml` fails the same way.

### Cause

The built-in bar is instantiated through `defaultBarComponent` (`shell/shell.qml`),
which sets its properties declaratively. A cloned bar goes through
`pluginBarLoader` instead, which loads it by URL:

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

`Bar.qml` declares three properties the host injects as `required`:

```qml
required property string omarchyPath
required property var barWidgetRegistry
required property var barConfig
```

A `Loader` using `source:` cannot satisfy `required property` — there is no way to
supply them at creation — so instantiation fails and the Loader goes to
`Loader.Error` before `configureBar()` ever runs. The shell logs:

```
WARN scene: file:///home/<user>/.config/omarchy/plugins/<user>.bar/Bar.qml[15:3]: Required property omarchyPath was not initialized
WARN scene: file:///home/<user>/.config/omarchy/plugins/<user>.bar/Bar.qml[17:3]: Required property barWidgetRegistry was not initialized
WARN scene: file:///home/<user>/.config/omarchy/plugins/<user>.bar/Bar.qml[22:3]: Required property barConfig was not initialized
```

Dropping `required` on those three (plain `property string omarchyPath: ""`,
`property var barWidgetRegistry: null`, `property var barConfig: null`) makes the
cloned bar load and render correctly — `configureBar()` assigns them immediately
after load. That looks like the right fix, and it would let any third-party bar
plugin work too, since they all face the same loader.

### Second bug: the real error is swallowed

The failure is very hard to diagnose because `pluginBarLoader`'s error handler
(`shell/shell.qml:256`) calls `errorString()`, which is not in scope there:

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

That throws, so instead of the "bar option ... failed to load, falling back to ..."
message the user only ever sees:

```
WARN scene: @shell.qml[256:-1]: ReferenceError: errorString is not defined
```

and `shell.failedBarId` is never set, so the documented fallback to the built-in
bar never happens either — hence no bar on screen at all rather than a degraded one.

### Steps to reproduce

1. `omarchy plugin clone omarchy.bar` (it switches `bar.id` to `<user>.bar`)
2. `omarchy restart shell`
3. No bar renders; `hyprctl layers | grep omarchy-bar` returns nothing.
4. `journalctl --user -e` shows the three "Required property" warnings and the
   `ReferenceError: errorString is not defined`.

### Expected

A freshly cloned bar renders identically to the built-in one — that is the point of
`omarchy plugin clone`. Failing that, the shell should fall back to the built-in bar
and log why.

### System

- Omarchy 4.0.2-1
- Arch Linux, Hyprland, single 1920x1080 display