Replace private imports (_select_device, _to_float32, SAMPLE_RATE, SPLIT_PATTERN)
from abogen.webui.conversion_runner with proper domain imports:
- select_device from abogen.domain.device
- to_float32, SAMPLE_RATE from abogen.domain.audio_helpers
- SPLIT_PATTERN defined locally (r'\n+')
Also verifies preview.py already uses domain imports correctly.
7 new tests in tests/test_domain_imports.py enforce the architecture rule.
