style(ux): add global theme selector

[selvalabs]

Summary

• add a shared theme selector to public, member and admin pages
• persist light/dark/system theme preference in localStorage
• apply dark theme through existing CSS variables on all three static surfaces

Validation

• uv run pytest tests/unit/test_web_home.py tests/unit/test_member_access.py::test_member_page_and_static_assets_are_served tests/unit/test_admin_dashboard.py::test_admin_dashboard_page_is_served_by_direct_url -q
• node --check src/monitor_comunitario/web/static/preferences.js
• node --check src/monitor_comunitario/web/static/app.js
• node --check src/monitor_comunitario/web/static/member.js
• node --check src/monitor_comunitario/web/static/admin.js
• uv run ruff check .
• uv run mypy src
• uv run pytest

Closes #60
Refs #25