Native recordings get no field labels from the accessibility tree (only the browser's do) #1868

At record time a typed field's label is kept as evidence (field_label on the event and on the compiled step; see packages/flow/DESIGN.md, "Record-time label capture"). Only PlaywrightBackend implements focused_field_label (openadapt_flow.backend.FieldLabelBackend). Windows UIA, macOS AX and Linux AT-SPI expose the focused field's name and labelled-by relation, but their backends don't implement the seam, so native recordings fall back to the nearest OCR line at compile time.

Close: implement focused_field_label on WindowsBackend (UIA LabeledBy, then Name), MacOSBackend (AXTitleUIElement, then AXTitle/AXDescription) and LinuxBackend (AT-SPI LABELLED_BY relation, then name). Read from the focused element only, never by position. Return None when there's no label, so the OCR fallback still applies.

Test: real apps on the macOS lab VM (Chart.swift note field) and the Linux AT-SPI CI job (chart_gtk.py), asserting that the compiled type step's field_label comes from the tree.