Add archived decision toggle and update count

What does this MR do?

The decision log panel now hides archived decisions by default. A "Show archived" toggle appears in the panel header when archived decisions exist. The decision counts in the panel header and on the icon button now count only active decisions.

How to test

1. Turn on the decision_log feature flag.
2. Open a work item that has decisions.
3. Open the decision log panel from the icon button in the work item header.
4. Archive a decision from the three dots menu on its card.
5. Verify the card disappears from the list.
6. Verify the count at the top and on the icon button both decrease by one.
7. Verify a "Show archived (1)" toggle appears.
8. Turn the toggle on.
9. Verify the archived card comes back with its "Archived" badge.
10. Verify the count does not change.
11. Turn the toggle off.
12. Verify the archived card disappears again.

References

• Closes #629493 (closed)
• Closes #629491 (closed)
• Design: https://www.figma.com/design/uJb85wjoRaj7MMvjxAmdHf/SDD-Iteration-Path?node-id=673-33393
• Follows up on the review of !255340 (merged)