PR3: napari widget button for plate overview

[ferrinm]

Part of #7. Depends on PR2.

What

Add a "Generate plate overview" button to PhenixDataLoaderWidget.

Behavior

• Button is disabled until an experiment is loaded
• Click → QFileDialog.getExistingDirectory() defaulting to the experiment directory
• Calls generate_plate_overview(experiment_path, chosen_dir) with sensible defaults (the function's defaults are designed to "just work" — no widget-side config in v1)
• On completion, opens the chosen directory in the OS file viewer:
  • macOS: subprocess.run(["open", dir])
  • Linux: subprocess.run(["xdg-open", dir])
  • Windows: subprocess.run(["explorer", dir])
• Status message shown in the widget's existing status area

Acceptance criteria

• Button visible in a sensible existing widget section (e.g. near the "Save data" button)
• Correctly enabled/disabled with the rest of the controls (see _set_controls_enabled in _widget.py:1062)
• pytest-qt test verifying button wiring: mock QFileDialog, mock the OS-open call, verify generate_plate_overview is invoked with the expected args
• All existing widget tests still pass

Out of scope

• Configuring overview parameters from the widget (advanced users call the function from a script)
• Progress feedback inside the napari window (terminal tqdm is enough for v1)
• Running in a background thread (the napari UI will block during overview generation; acceptable for a diagnostic action)

[ferrinm]

Implemented in #13.