Edit mode now exposes a project picker so a deadline or appointment can be
moved to a different matter. Backend Update accepts project_id (and
clear_project for appointments), validates visibility on the destination,
and emits *_project_changed audit rows on both the OLD and NEW project so
each side's Verlauf still shows the move.
Personal-to-project linking and project-to-personal unlinking are gated by
the existing personal-Appointment creator check; project-to-project moves
re-use the existing requireMutationRole gate plus a fresh visibility check
on the target.
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