feat: add start.sh for Linux/macOS, harden both launchers

start.sh mirrors start.bat: finds Python 3.10+, creates the venv,
installs PyTorch (CUDA index on Linux, the MPS build on macOS, or the CPU
index with --cpu), installs ACE-Step, reports the device and launches the
UI. Same flags, and the defaults can also come from the environment
(PORT=7870 ./start.sh). On macOS it passes --bf16 false, which the README
already calls for. Dropped start.sh from .gitignore, where it sat among
the upstream author's local scratch files.

Both launchers also gain two fixes found while testing on WSL:

- A venv is only accepted if pip works in it, not merely if the
  interpreter exists. A directory left by an interrupted install looked
  ready and then failed several steps later with a misleading "check your
  internet connection". Such a venv is now recreated, and if creation
  fails on Debian/Ubuntu the error points at python3-venv, which is the
  actual cause there.
- The launch banner announced the URL as if the server were already up,
  while model loading still had a minute to go. It now says the interface
  will be available once "Running on local URL" appears.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -200,5 +200,4 @@ ui/components_demo.py
 data_sampler_demo.py
 pipeline_ace_step_demo.py
 *.wav
-start.sh
 exps/*