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/api/v1/ai/meal-plan/generate/route.ts

@@ -101,7 +101,7 @@ export async function POST(req: NextRequest) {
   if (!mealPlan) {
     const planId = crypto.randomUUID();
     await db.insert(mealPlans).values({ id: planId, userId, weekStart: parsed.data.weekStart });
-    mealPlan = { id: planId, userId, weekStart: parsed.data.weekStart, createdAt: new Date() };
+    mealPlan = { id: planId, userId, weekStart: parsed.data.weekStart, isPublic: false, createdAt: new Date() };
   }
 
   const createdEntries: Array<{ id: string; day: string; mealType: string; recipeId: string; recipeTitle: string }> = [];