Allow reordering custom metrics sources

[noponoponopo]

Context of Contribution

• Bug Fix
• Refactoring
• New Feature
• Others

Summary of the Proposal

Allow users to reorder custom metrics sources by dragging the handle in Metrics settings.

The new order is persisted in the existing custom metrics configuration and immediately applied to custom metrics displays.

Reason for the new feature

Users with multiple custom metrics sources currently need to remove and re-add sources to change their display order.

This change uses the existing source array as the display order, adds no new settings or dependencies, and follows the drag-and-drop pattern already used by the custom runner frame editor.

Verification

• Confirmed drag handles use the full row height as their hit area.
• Confirmed surrounding rows move to preview the destination while dragging.
• Confirmed the dragged row is restored after dropping.
• Confirmed the reordered sources persist and emit a configuration change.
• 166 tests passed on macOS arm64.
• Release build succeeded.

Checklist

• I have read the CONTRIBUTING.md and agree to follow it.
• This PR does not contain commits of multiple contexts.
• Code follows proper indentation and naming conventions.
• Implemented using only APIs that can be submitted to the App Store.

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[Kyome22]

@noponoponopo
I really appreciate the hard work you have put into the trial and error of this implementation. However, regarding the sorting functionality, I already have the relevant expertise and existing code assets, so it would be more efficient for me to implement it myself. Your proposal is valid, so I would appreciate it if you could create an issue for it instead of a Pull Request.

As for the code, while the general implementation approach is correct, there are quite a few parts I would like to revise.