Fix grouped shapes disappearing after image changes

[vietanhdev]

Fixes #103.

Root cause

Grouping and ungrouping changed group_id values only in memory. The canvas did not emit its modification signal, so the labeling widget stayed clean and did not save before changing images. The edits also had no undo snapshot.

Changes

• record Group/Ungroup edits in the canvas undo history
• emit the existing change signal only when group IDs actually change
• add regression coverage for dirty notification, undo, whole-group ungrouping, and JSON save/load persistence

Validation

• QT_QPA_PLATFORM=offscreen .venv/bin/python -m unittest discover -s tests -v: 140 passed, 13 optional-model tests skipped
• ruff check anylabeling/views/labeling/widgets/canvas.py tests/test_canvas_grouping.py
• manual offscreen workflow: group → dirty signal → save → reload; group IDs [1, 1] persisted and undo was available