Increase max-width of duration input to 5rem

[jhimross]

Fixes #1762

How to test the changes in this Pull Request:

1. Check out this branch.
2. Go to WordCamp → Sessions in the WordCamp admin.
3. Create or edit a session.
4. Open the Session Info panel.
5. Verify that the Hours and Minutes number input fields are wider and easier to use.
6. Confirm that the increment/decrement controls continue to function correctly.
7. Verify there are no layout regressions on different screen sizes.

Before

The number input fields are cramped due to a max-width of 3rem.

After

The wider (5rem) input fields provide improved readability and usability while maintaining the existing layout.