feat: allow future transaction dates

[Louise-Qiuqiu]

Summary

Allow transaction creation/editing flows to select dates beyond today so users can pre-record known future expenses or income.

Changes

• remove the DateTime.now() upper bound when opening the date picker in AmountEditorSheet
• keep using the shared wheel pickers' default max date, which already allows future dates
• this also applies to transfer records because they reuse the same sheet

Testing

• verified the date restriction came from AmountEditorSheet
• confirmed both date-only and date-time pickers no longer receive a maxDate of today
• Flutter tooling is not installed in this execution environment, so I could not run flutter test / flutter analyze locally

Fixes #212