Refactor: split canvas/workers, add constants, deduplicate shared logic

Refactor: extract LabelModeController, add scan/export, fix overlays and session restore

Fix scan overlay and model dialog initial dir

Add Model Check scan mode, mode-gated UI, fix dialog dirs, and update README

Add class filter, frame jump, conf cache, clean labels, export options, and refactor playback/inference

Add Video Capture and Model Check modes with nav rename and UI cleanup

Add segmentation mode, nav column, toolbar, and safe label handling

Add Check Mode editing, undo/redo, delete image, and label count stats

Fix Auto Annotate stability with batched QThread and polling UI

Add Auto Annotate and restructure into app/ package

Initial commit: LabelItem custom tool