Fix QLedLine QHBoxLayout::setMargin() deprecation for Qt6

- Removed redundant setMargin(0) call (setContentsMargins already used)
- QHBoxLayout::setMargin() removed in Qt6

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

AGENTS.md

@@ -0,0 +1,4 @@
+<!-- entire-agent:begin -->
+Read .entire/agent-guide.md for this repository's workflow, source inspection, and verification guidance.
+@.entire/agent-guide.md
+<!-- entire-agent:end -->