fix(parent-complete): clear today's assignee for rotation chores

[tempus2016]

Summary

Clicking Parents did it on a rotation-mode chore (alternating / random / balanced) showed a success toast but left the chore visually assigned to the same child for the rest of the day — both on the child's stats card and in the Manage Chores Current column.

Root cause

Two gaps in the parent-completion path:

1. coord_assignments.py _is_rotation_done_today filtered with if comp.child_id not in pool: continue. The __parent__ sentinel id is never in the rotation pool, so a parent-completion didn't count toward daily_limit and the chore stayed visible on the active child's list. This contradicted the docstring above the call site in coord_chores.py which already noted that parent completions should retire the chore for the whole pool.
2. coord_chores.py async_parent_complete_chore wrote a completion + per-child last_completed, but never cleared chore.assignment_current_child_id. The midnight refresh recomputes it next day, but until then the Current column kept pointing at the original child.

Fix

• _is_rotation_done_today: also count comp.child_id == "__parent__" toward the daily quota.
• async_parent_complete_chore: clear chore.assignment_current_child_id for non-everyone modes and persist via update_chore. The midnight refresh restores it normally for the next day.

Everyone-mode chores are untouched (they don't cache a current assignee).

Tests

• Renamed test_parent_complete_does_not_change_rotation_pointer → test_parent_complete_clears_rotation_pointer_today and flipped the assertion. The old name captured the broken behavior, not the desired one.
• Added test_parent_complete_preserves_everyone_mode_pointer (no-op for everyone-mode).
• Added test_parent_complete_counts_toward_rotation_done_today (asserts _is_rotation_done_today flips to True after a parent completion).
• Full suite: 454 passed. Ruff: clean.

Test plan

• Manage Chores → pick a chore in alternating/random/balanced mode. Confirm Current column shows a child.
• Click Parents did it (👤✓), accept the confirmation.
• Current column now shows —.
• The chore disappears from the child's stats card / active list for the rest of the day.
• After next midnight, the rotation recomputes normally (same alternating cadence as if the chore had not been completed by a child).

Fixes #351