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/recipe/substitute-ingredient-popover.tsx

@@ -33,20 +33,28 @@ export function SubstituteIngredientPopover({
   const [loading, setLoading] = useState(false);
   const [substitutions, setSubstitutions] = useState<Substitution[]>([]);
   const [fetched, setFetched] = useState(false);
+  const [error, setError] = useState<string | null>(null);
 
   async function fetchSubstitutions() {
     if (fetched) return;
     setLoading(true);
+    setError(null);
     try {
       const res = await fetch("/api/v1/ai/substitute", {
         method: "POST",
         headers: { "Content-Type": "application/json" },
         body: JSON.stringify({ ingredient, recipeTitle }),
       });
-      if (!res.ok) return;
+      if (!res.ok) {
+        const body = await res.json().catch(() => null) as { error?: string } | null;
+        setError(body?.error ?? "Couldn't fetch substitutes.");
+        return;
+      }
       const data = await res.json() as { substitutions: Substitution[] };
       setSubstitutions(data.substitutions);
       setFetched(true);
+    } catch {
+      setError("Couldn't fetch substitutes.");
     } finally {
       setLoading(false);
     }
@@ -91,6 +99,9 @@ export function SubstituteIngredientPopover({
             ))}
           </div>
         )}
+        {!loading && error && (
+          <p className="text-sm text-destructive py-1">{error}</p>
+        )}
         {!loading && fetched && substitutions.length === 0 && (
           <p className="text-sm text-muted-foreground py-1">No substitutions found.</p>
         )}