Mirrors the existing recipe public-share pattern (/r/[id], gated by a
visibility flag, no auth required) — shopping lists previously only
supported private per-user collaborator invites (email + viewer/editor
role), with no way to hand someone a link who isn't already a member.
- shoppingLists.isPublic (owner-only toggle, same access-control pattern
already used for renaming/deleting the list).
- New public /s/[id] route added to PUBLIC_PATHS — read-only aisle-grouped
view with a signup CTA for logged-out visitors, styled after /r/[id].
- The existing collaborator-invite dialog (ShareShoppingListButton) now
also has a "Public link" toggle + copy-link button at the top, so
there's one Share entry point for both private and public sharing.
Verified locally: toggling public via the real dialog UI persists
correctly (confirmed via DB + a fresh curl PATCH to rule out a client
race in my own test script), and the public link loads with zero auth
for a logged-out browser context.
Arnaud