fix(install): uv fallback via pip and flash-attn install targets venv

[dipeshbabu]

This PR makes the install flow resilient in restricted environments and fixes flash-attn wheel installation into the correct Python environment.

Changes

• scripts/install.sh: if the curl-based uv installer fails (common under SSL interception / blocked GitHub), fall back to python -m pip install --user uv and ensure PATH is updated.
• scripts/install_flash_attn.py: run uv pip install --python sys.executable ... so flash-attn installs into the TuFT venv that invoked the script (prevents “No virtual environment found”).

Why

• Users behind restricted networks can’t always run curl ... | sh successfully.
• uv pip install without --python can fail when no active venv is detected, even when TuFT created one.

Testing

• bash -n scripts/install.sh
• python -m py_compile scripts/install_flash_attn.py (or uv run python -m py_compile ...)