Replace FTimeField/FDateField format DateFormat with callback

[Pante]

Describe the changes

• Change FTimeField.picker(format: ...) and FDateField.calendar(format: ...) from DateFormat? to String Function(BuildContext, FTime/DateTime, DateFormat) — the callback receives the locale-aware default DateFormat so callers can compose or fall back to it.
• Add FTimeField.defaultFormat and FDateField.defaultFormat static functions used as the new non-nullable defaults (mirrors FDateTimePicker.defaultDateBuilder / FDateField.defaultIconBuilder).
• Update existing custom format tests, docs_snippets usages, and CHANGELOG.

Checklist

• I have read the CONTRIBUTING.md.
• I have included the relevant unit/golden tests.
• I have included the relevant samples.
• I have updated the documentation accordingly.
• I have added the required flutters_hook for widget controllers.
• I have updated the CHANGELOG.md if necessary.

[Copilot]

Pull request overview

This PR updates FTimeField.picker(format: ...) and FDateField.calendar(format: ...) to accept a formatting callback instead of a DateFormat?, enabling callers to compose/customize output while receiving the locale-aware default DateFormat.

Changes:

• Introduce FTimeField.defaultFormat / FDateField.defaultFormat as new defaults for the formatter callback.
• Update picker/calendar implementations to call the formatter callback with the computed locale-aware DateFormat.
• Update tests, docs snippets, and CHANGELOG for the breaking API change.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
forui/lib/src/widgets/time_field/time_field.dart	Adds defaultFormat and changes picker factory format parameter type to a callback.
forui/lib/src/widgets/time_field/picker/picker_time_field.dart	Switches internal formatting logic to call the callback (defaulting to FTimeField.defaultFormat).
forui/lib/src/widgets/date_field/date_field.dart	Adds defaultFormat and changes calendar factory format parameter type to a callback.
forui/lib/src/widgets/date_field/calendar/calendar_date_field.dart	Switches internal formatting logic to call the callback (defaulting to FDateField.defaultFormat).
forui/test/src/widgets/time_field/picker/picker_time_field_test.dart	Updates the “custom format” test to the new callback API.
forui/test/src/widgets/date_field/calendar/calendar_date_field_test.dart	Updates the “custom format” test to the new callback API.
docs_snippets/lib/usages/widgets/time_field.dart	Updates snippet to use FTimeField.defaultFormat and removes intl import.
docs_snippets/lib/usages/widgets/date_field.dart	Updates snippet to use FDateField.defaultFormat.
forui/CHANGELOG.md	Documents the breaking change and the new default formatter helpers.

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