fix: substitute lookup, zero-quantity display, chatbot close button + markdown

- Substitute finder never applied the user's BYOK/admin AI key (only fell
  back to raw process.env), so it silently failed whenever the key was
  stored via settings rather than a literal env var. Now resolves via
  getDefaultProviderWithKey like every other AI route. Popover also
  surfaces real error messages instead of swallowing failures.
- Ingredients with quantity 0 (salt, pepper, "to taste") rendered the
  literal digit "0" in cooking mode, print view, public recipe page,
  shopping lists, and serving scaler — several sites relied on generic
  truthiness/filter(Boolean), which doesn't catch a stored "0" string.
  Added a shared hasQuantity() helper and applied it everywhere quantity
  is rendered, plus in the AI recipe-chat context sent to the model.
- Recipe chat panel rendered a duplicate close button on top of shadcn
  Sheet's own built-in close X, producing a garbled overlapping glyph.
  Removed the duplicate.
- Recipe chat assistant replies are markdown from the model but were
  rendered as raw text; added react-markdown so formatting actually
  renders.

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

apps/web/components/meal-plan/shopping-list-view.tsx

@@ -5,6 +5,7 @@ import { cn } from "@/lib/utils";
 import { Check, Package, Loader2 } from "lucide-react";
 import { toast } from "sonner";
 import { Button } from "@/components/ui/button";
+import { hasQuantity } from "@/lib/fractions";
 
 type Item = {
   id: string;
@@ -104,9 +105,9 @@ export function ShoppingListView({
                 <span className={cn("flex-1 text-sm", item.checked && "line-through text-muted-foreground")}>
                   {item.rawName}
                 </span>
-                {(item.quantity || item.unit) && (
+                {(hasQuantity(item.quantity) || item.unit) && (
                   <span className={cn("text-xs text-muted-foreground tabular-nums shrink-0", item.checked && "opacity-50")}>
-                    {item.quantity}{item.unit ? ` ${item.unit}` : ""}
+                    {hasQuantity(item.quantity) ? item.quantity : ""}{item.unit ? ` ${item.unit}` : ""}
                   </span>
                 )}
               </button>