refactor: centralize test fixtures + macOS APFS disk usage fix

- Add tests/cli/conftest.py and expand tests/gui/conftest.py with full
  fixture hierarchies mirroring the production DI chain — mock_device_info
  → mock_device_svc → lcd → mock_connect_lcd, mock_led_svc → led →
  mock_connect_led, factory fixtures for themes/devices/metrics
- Remove duplicate fixture/helper functions from test_display.py,
  test_led.py, test_theme.py, test_device.py, test_system.py,
  test_lcd_handler.py, test_trcc_app.py — single source of truth in conftest
- Fix macOS APFS disk usage: psutil.disk_usage('/') reports ~3% on macOS
  due to snapshot volumes; use diskutil apfs list to read actual container
  usage, fall back to psutil on error

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>