feat: share shopping lists via a public link

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.

apps/web/proxy.ts

@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getSessionCookie } from "better-auth/cookies";
 
-const PUBLIC_PATHS = ["/login", "/signup", "/verify-email", "/forgot-password", "/reset-password", "/api/auth", "/r/", "/u/", "/docs", "/api/v1/openapi.json", "/api/webhooks", "/api/v1/invites/", "/api/internal/"];
+const PUBLIC_PATHS = ["/login", "/signup", "/verify-email", "/forgot-password", "/reset-password", "/api/auth", "/r/", "/u/", "/s/", "/docs", "/api/v1/openapi.json", "/api/webhooks", "/api/v1/invites/", "/api/internal/"];
 const ADMIN_PATHS = ["/admin"];
 
 export async function proxy(request: NextRequest) {