fix(deps): use uv tool install for playwright-py; gitignore node_modules

Two fixes rolled up:

1. Add .gitignore with **/node_modules/, package-lock.json, Python venv /
   cache artifacts, and OS metadata. Prior make deps run produced a 603-
   file playwright-js/node_modules tree that should never be tracked.

2. Makefile deps target: install playwright-py via `uv tool install
   playwright` instead of `pip install --system`. Earlier attempts with
   pip --user, pip --system, and uv pip --system all failed on externally-
   managed Python (PEP 668 on Arch). `uv tool install` creates an isolated
   venv for the CLI, avoiding the conflict. Chromium browsers are shared
   with the JS side via ~/.cache/ms-playwright — no re-download.

   Also added uv itself to the deps target (was missing).

   Library import (`import playwright`) still requires per-project venv,
   which is the right pattern on externally-managed systems. Deps output
   mentions this explicitly.

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