Source data_dir from DATA_DIR env var instead of hard-coding path

[Copilot]

tts.yaml hard-codes data_dir to a machine-specific path, breaking runs on any other environment.

• Move data_dir into config/paths/default.yaml using ${oc.env:DATA_DIR}, consistent with the existing root_dir pattern
• Remove the hard-coded paths.data_dir override from config/tts.yaml

Users set the data directory via environment:

export DATA_DIR=/path/to/data

Or CLI override:

uv run python ./train.py paths.data_dir=/path/to/data

OmegaConf raises a clear InterpolationKeyError if DATA_DIR is unset.