refactor(schema): drop legacy pre-2.7 workout session columns (#1310)

[pankaj-basnet]

PR Description

Removes the deprecated pre-2.7 legacy columns (date, time_start, time_end) from the local Drift schema, PowerSync table declarations, and the WorkoutSession model mapping.

• added code in fix(sessions): resolve cross-midnight split bug by migrating to full DateTime (wger-project/wger#2379) #1248, these legacy fields are no longer needed for database reconstruction and can be safely dropped from client-side sync definitions.

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Proposed Changes

1. lib/database/powersync/tables/routines.dart

• Removed date, timeStart, and timeEnd column getters from WorkoutSessionTable.
• Updated PowersyncWorkoutSessionTable sync declaration to drop date, time_start, and time_end from the column list.

2. lib/features/routines/models/session.dart

• Removed legacy column entries (date, timeStart, timeEnd) from toCompanion().

3. lib/database/powersync/database.g.dart

• Regenerated via dart run build_runner build --delete-conflicting-outputs to update generated companions and table definitions.

Related Issue(s)

• Closes Remove drift compatibility for workout sessions #1310
• Follow-up to fix(sessions): resolve cross-midnight split bug by migrating to full DateTime (wger-project/wger#2379) #1248

Please check that the PR fulfills these requirements

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