This change fixes a problem where showDateRangePicker ignored the dayShape property in DatePickerThemeData, leading to days always rendering as circles regardless of the theme. This was caused by the internal _DayItem widget using a hardcoded BoxDecoration regardless of the theme configuration.

Fixes #181500

Before	After

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.