feat: anonymous read-only public link + QR code for meal plans (v0.68.0)

Mirrors shopping lists' isPublic pattern but read-only only -- no
publicEditable equivalent, since anonymous editing of someone's
weekly meal schedule isn't a real use case the way collaborative
shopping-list editing is. New PATCH /api/v1/meal-plans/{weekStart}
toggles it (lazily creates the week's plan row if missing, same
pattern as the existing POST). Public page at /m/[id] renders the
week grouped by day, linking through to public/unlisted recipes and
just showing the title otherwise.

ShareMealPlanButton gained the same public-link toggle UI as its
shopping-list sibling, plus a QR code (generated client-side via the
already-installed `qrcode` package) for the link once enabled --
handing someone a printed or screenshotted plan doesn't require
typing a URL.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/(app)/meal-plan/page.tsx

@@ -136,7 +136,7 @@ export default async function MealPlanPage({
           </div>
         </div>
         <div className="flex flex-wrap items-center gap-2">
-          <ShareMealPlanButton weekStart={weekStart} />
+          <ShareMealPlanButton weekStart={weekStart} mealPlanId={plan?.id} initialIsPublic={plan?.isPublic ?? false} />
           <NewShoppingListButton
             defaultWeekStart={weekStart}
             defaultName={formatMessage(msgs.mealPlan.shoppingListWeekName, { week: label })}