fix(window): scope taskbar drag clamping to runtime placements

[CodeZeno]

Summary

• Restrict taskbar drag clamping to runtime-created drag placements so authored theme placements retain their configured positions near the tray.
• Add regression coverage for authored placements and ensure the runtime-only flag is not serialized into themes.

Testing

• Not run; tests were not included in the provided change details.

Checklist

• I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md.
• The change is focused and contains no credentials or sensitive data.
• I have added or updated relevant documentation and tests, where applicable.
• I have left changelog and version changes to maintainers unless requested.