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/app/api/v1/ai/substitute/route.ts

@@ -3,6 +3,7 @@ import { z } from "zod";
 import { requireSession } from "@/lib/api-auth";
 import { checkAndIncrementTierLimit, TierLimitError } from "@/lib/tiers";
 import { applyRateLimit } from "@/lib/rate-limit";
+import { getDefaultProviderWithKey } from "@/lib/ai/resolve-user-key";
 import { substituteIngredient } from "@/lib/ai/features/substitute-ingredient";
 
 const Schema = z.object({
@@ -36,10 +37,14 @@ export async function POST(req: NextRequest) {
     ? `recipe "${parsed.data.recipeTitle}"`
     : "a general recipe";
 
+  const aiConfig = parsed.data.provider
+    ? { provider: parsed.data.provider, model: parsed.data.model }
+    : await getDefaultProviderWithKey(session!.user.id);
+
   const substitutions = await substituteIngredient(
     parsed.data.ingredient,
     context,
-    { provider: parsed.data.provider, model: parsed.data.model }
+    aiConfig
   );
 
   return NextResponse.json({ substitutions });