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)

static/style.css

@@ -257,6 +257,36 @@ body {
 .lang-active { background: var(--primary); color: #fff; }
 .lang-active:hover { background: #0d47a1; color: #fff; }
 
+/* ── Public page ── */
+.public-page .navbar {
+    justify-content: space-between;
+}
+.public-heading {
+    margin-bottom: 16px;
+}
+.public-footer {
+    text-align: center;
+    color: var(--muted);
+    font-size: 0.8rem;
+    margin-top: 32px;
+}
+.public-url-box {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    background: #f5f5f5;
+    border: 1px solid var(--border);
+    border-radius: 4px;
+    padding: 6px 10px;
+    margin-top: 8px;
+    word-break: break-all;
+}
+.public-url-box code {
+    flex: 1;
+    font-size: 0.8rem;
+    color: var(--text);
+}
+
 /* ── Responsive ── */
 @media (max-width: 600px) {
     .navbar {