Support uv for environment setup and dependency install

[Raymondycp]

Summary

Allow the LLama GUI venv to be created and its dependencies installed with
uv when it is available, while keeping the
existing python -m venv + pip flow as a fallback.

Changes

• install.sh: detect uv on PATH; create the venv with
  uv venv --seed --python "$PY_CMD" and install requirements with
  uv pip install --python "$VENV_PYTHON" -r requirements.txt. Falls back to
  python -m venv / pip when uv is absent.
• backend/services/git_update.py: new dependency_install_command()
  used by the in-app updater (install_python_dependencies). Prefers
  uv pip install --python <sys.executable> -r requirements.txt when uv is on
  PATH, else the previous sys.executable -m pip install -r requirements.txt.
• AGENTS.md / CONTRIBUTING.md: document the uv workflow, and why
  --seed / pip-in-venv must be preserved for install.sh and the updater.

Why uv + --seed

• uv venv without --seed produces a venv with no pip; both
  install.sh and the in-app updater assume pip exists in the venv.
• There is no pyproject.toml, so dependencies are managed with
  uv pip install -r requirements.txt (not uv sync).

Tests

Ran with the project venv:

• python -m unittest discover tests -v — 825 tests OK (includes two new
  tests covering the uv-preference and pip-fallback branches.
• tests/backend/test_docs_links.py and test_docs_sync.py — OK.

[thomas9120]

small clarification needed regarding windows install via bat file, it wouldn't use uv but the in-app updater would on Windows. I can make these changes post merge.