bar: add custom entries

[ratataque]

Summary

• Add support for custom bar entries loaded from bar.entries[].source.
• Preserve custom entry source values when saving taskbar settings from the control center.

Usage

Users can add a custom bar item in ~/.config/caelestia/shell.json:

{
  "id": "custom",
  "enabled": true,
  "source": "file:///home/user/.config/caelestia/components/MyWidget.qml"
}

Verification

• Ran python3 scripts/qml-lint-conventions.py modules/bar/Bar.qml modules/controlcenter/taskbar/TaskbarPane.qml
• Ran git diff --check
• Ran cmake --build build
• Runtime-tested with a config-side custom ZMK battery widget.

[Copilot]

Pull request overview

This PR extends the bar/taskbar configuration so a bar entry can load a user-provided QML component via a bar.entries[].source URL, and ensures the control center doesn’t drop that source field when re-saving taskbar settings.

Changes:

• Add a "custom" bar entry delegate that loads its UI via Loader.source resolved from bar.entries[].source.
• Update the control center taskbar pane model and save logic to round-trip an entry’s source value.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
modules/controlcenter/taskbar/TaskbarPane.qml	Persist entries[].source through the control center’s entries model and save path.
modules/bar/Bar.qml	Add helper functions and a DelegateChoice for "custom" entries to load external QML via source.

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