Run browser tests locally: Playwright 1.64.1 in .venv, scripts switch to it on their own

The browser tests and screenshot tools now work with plain python3 on a machine whose
system Python refuses pip installs: tools/use_venv.py re-runs them with the repo's .venv.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/shot.py

@@ -39,6 +39,7 @@ Example: a belt carrying a heap of sand, in the smooth look:
 import argparse, pathlib, sys
 sys.path.insert(0, str(pathlib.Path(__file__).resolve().parent))
 from where import translate
+from use_venv import ensure_playwright; ensure_playwright()
 from playwright.sync_api import sync_playwright
 
 ROOT = pathlib.Path(__file__).resolve().parent.parent