Adds `e2e` subcommand to manage.sh supporting headless (default) and
headed (E2E_HEADLESS=false via xvfb-run) Playwright test runs with
per-mode JSON report output under tests/e2e/results/<mode>/.
setup.sh now installs make (via apt/dnf/pacman/brew) before git and
Docker so that manage.sh commands work out of the box on minimal server
installs. manage.sh adds a preflight guard that catches a missing make
early and redirects the user to ./manage.sh setup. Also fixes the
post-setup next-steps hint to use ./manage.sh instead of bare make.
Makefile restart target now runs compose down before preflight so ports
are free when preflight assigns them; previously preflight ran first while
the old container still held 8502, causing it to bump to 8503.
manage.sh start/restart/open now read STREAMLIT_PORT from .env instead
of re-running preflight after startup (which would see the live container
and bump the reported port again).
