Better handling of time zones

[rolandgeider]

Proposed Changes

We now send the current time zone to the server when serializing datetime objects. This was causing entries to be saved some hours wrong or depending on the time, on a different day.

Related Issue(s)

Closes #576

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• Tests for the changes have been added (for bug fixes / features)
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  (run dart format .)
• Updated/added relevant documentation (doc comments with ///).
• Added relevant reviewers.

[Copilot]

Pull Request Overview

This PR improves timezone handling in the wger Workout Manager app by ensuring datetime objects are properly serialized with UTC timezone information to prevent incorrect date/time storage on the server.

• Introduces a new dateToUtcIso8601 function that converts DateTime objects to UTC before serialization
• Updates all model classes to use the new timezone-aware serialization
• Refactors date/time utility functions into a dedicated date.dart helper file

Reviewed Changes

Copilot reviewed 29 out of 29 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lib/helpers/date.dart	New helper file containing date/time utility functions moved from misc.dart
lib/helpers/json.dart	Adds new dateToUtcIso8601 function for timezone-aware serialization
lib/models/nutrition/log.dart	Updates datetime field to use UTC serialization
lib/models/workouts/log.dart	Updates date field to use UTC serialization
lib/models/workouts/routine.dart	Updates created field to use UTC serialization
lib/models/nutrition/nutritional_plan.dart	Updates creationDate field to use UTC serialization
lib/widgets/nutrition/forms.dart	Refactors date/time parsing and updates function names
lib/screens/log_meal_screen.dart	Improves date/time handling and UI responsiveness

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