feat: add basic webui matching stable-audio CLI options

[IsaacNewtonne]

Summary:
Adds a minimal Gradio WebUI that maps directly to the main stable-audio CLI options, intended as a simple UI path for users who want CLI-parity controls.

What’s included
- New basic UI module:
  - stable_audio_3/interface/basic_cli_webui.py
- New launcher:
  - run_basic_webui.py
- README quick-start addition for launching the basic web UI.

Exposed controls (CLI parity focus)
- Model/runtime: --model, --device, --no-half
- Generation: --prompt, --negative-prompt, --duration, --steps, --cfg-scale, --seed, --batch-size
- Audio-to-audio: --init-audio, --init-noise-level
- Inpainting/continuation: --inpaint-audio, --inpaint-start, --inpaint-end (supports comma-separated multi-region input in UI)
- Decode/LoRA: chunked decode mode, --lora-ckpt-path, --lora-strength, --lora-index
- Output: WAV export/download

Validation
- python -m compileall stable_audio_3/interface/basic_cli_webui.py run_basic_webui.py

[mhrice]

Thanks for this contribution! I wonder if you think it would just be better to merge this into the existing gradio ui?
It seems like the only main features that we've missing there is multiple inpainting, model switching, and batch size?