feat: public sharing links for households

Add hard-to-guess public URLs (/public/<token>) that show a read-only
meal status dashboard for each household. Only people with the link can
see the data — no login required.

- models.py: public_token + public_enabled columns on households,
  get_household_by_public_token(), generate_public_token(),
  set_public_enabled()
- meals.py: /public/<token> route, /admin/public-link for
  generate/disable/enable/regenerate
- templates/public.html: clean read-only public dashboard
- templates/public-not-found.html: 404 for invalid/disabled links
- templates/dashboard.html: admin UI with copy/regenerate/disable
- static/style.css: public page and URL box styles
- i18n.py: 18 new translation keys (en/fr/de)

templates/public-not-found.html

@@ -0,0 +1,7 @@
+{% extends "base.html" %}
+{% block title %}{{ t('Not Found — Meal Tracker') }}{% endblock %}
+{% block content %}
+<h2>🔗 {{ t('Link Not Found') }}</h2>
+<p class="empty-state">{{ t("This sharing link is invalid or has been disabled. Please ask the household admin for a new link.") }}</p>
+<p class="empty-state" style="margin-top: 2rem;"><a href="{{ url_for('auth.login') }}">{{ t('Log in') }}</a></p>
+{% endblock %}