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/recipe-chat/route.ts

@@ -7,6 +7,7 @@ import { getModelConfigForUseCase } from "@/lib/ai/resolve-user-key";
 import { resolveModel } from "@/lib/ai/factory";
 import { db, recipes, eq, and } from "@epicure/db";
 import { getUserPrivateBio, buildUserBioContext } from "@/lib/ai/user-bio";
+import { hasQuantity } from "@/lib/fractions";
 
 const Schema = z.object({
   recipeId: z.string().uuid(),
@@ -39,7 +40,7 @@ export async function POST(req: NextRequest) {
   }
 
   const ingredientList = recipe.ingredients
-    .map((ing) => [ing.quantity, ing.unit, ing.rawName, ing.note ? `(${ing.note})` : ""].filter(Boolean).join(" "))
+    .map((ing) => [hasQuantity(ing.quantity) ? ing.quantity : null, ing.unit, ing.rawName, ing.note ? `(${ing.note})` : ""].filter(Boolean).join(" "))
     .join("\n");
 
   const stepList = recipe.steps