fix(sessions): resolve cross-midnight split bug by migrating to full DateTime (wger-project/wger#2379)

[pankaj-basnet]

still WIP

• TODOs: fix test files

PR Title

fix(sessions): resolve cross-midnight split bug by migrating to full DateTime (#2379)

PR Description

Overview
This PR resolves the persistent "midnight session split" bug (Issue #2379). Previously, workout sessions were tracked using TimeOfDay, which inherently strips date context. This caused duration calculation failures and split active sessions when a user worked out across midnight.

Architecturally, this PR migrates the entire session time-tracking pipeline—from the Drift database layer up to the Riverpod providers and UI forms—to use fully qualified DateTime objects (UTC ISO8601 strings in the database).

Key Architectural Changes

• Time Tracking Migration: Replaced TimeOfDay with DateTime for timeStart and timeEnd to natively support date-spanning logic.
• Intelligent Lazy Sessions: Introduced a sessionMaxDuration (5 hours) constant. The WorkoutLogRepository now accurately queries for open sessions within this rolling time window rather than relying on static calendar-day boundary checks (isSameDayAs).
• Cross-Midnight UI Logic: The SessionForm now automatically detects if an end time is chronologically earlier than a start time (e.g., 23:00 to 01:00) and safely adds a 1-day offset to the DateTime object.

File Modifications

#	Code File Changed	Description of Code Changes
1	lib/database/converters/date_only_text_converter.dart	Created DateTimeTextConverter to serialize/deserialize DateTime objects to ISO8601 strings for SQLite.
2	lib/database/powersync/tables/routines.dart	Updated WorkoutSessionTable schema columns (time_start, time_end) to leverage the new DateTimeTextConverter.
3	lib/helpers/routines/validators.dart	Updated validateWorkoutSessionTimes signature to accept DateTime parameters instead of TimeOfDay.
4	lib/models/workouts/session.dart	Migrated timeStart and timeEnd model properties. Added sessionMaxDuration constant. Refactored duration and string formatter getters for null-safety.
5	lib/providers/gym_state.dart	Migrated GymModeState startTime to DateTime, defaulting to clock.now().
6	lib/providers/workout_logs_repository.dart	Refactored addLocalDrift lazy session creation. It now checks for sessions within the 5-hour window and explicitly creates new sessions if the previous matching session is already closed (timeEnd != null).
7	lib/widgets/dashboard/calendar.dart	Adapted the UI layer to extract TimeOfDay from the new DateTime models for correct visual rendering.
8	lib/widgets/routines/forms/session.dart	Added null-safe, local state handling for time inputs. Implemented the end.isBefore(start) check to gracefully add a 24-hour offset for midnight-crossing logs.
9	lib/widgets/routines/gym_mode/session_page.dart	Overhauled the active session lookup query. It now finds unclosed sessions within sessionMaxDuration instead of strictly checking if the session shares the exact calendar date.

Related Issues

• Resolves Training over night breaks routine into two wger#2379

Related Issue(s)

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• Tests for the changes have been added (for bug fixes / features)
• Set a 100-character limit to avoid white space diffs (run dart format .)