feat: add skip habit endpoint

[thomasluizon]

Summary

• New POST /api/habits/{id}/skip endpoint
• Advances a recurring habit's due date to the next scheduled occurrence without creating a HabitLog
• Reuses existing Habit.AdvanceDueDate(today) domain method

Changes

• SkipHabitCommand - MediatR command + handler: validates habit is recurring, due/overdue, not completed, then advances due date
• SkipHabitCommandValidator - FluentValidation: UserId/HabitId not empty
• HabitsController - new POST {id}/skip action returning 204 No Content

Frontend dependency

• orbit-ui PR fix: recurring habits match by title only in calendar dedup #42

Test plan

• Skip a daily habit due today - verify DueDate advances to tomorrow
• Skip an overdue habit (3 days late) - verify DueDate advances past today
• Try to skip a one-time task - verify 400 error
• Try to skip a future habit - verify 400 error
• Try to skip a completed habit - verify 400 error

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