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/print/[id]/page.tsx

@@ -3,6 +3,7 @@ import { headers } from "next/headers";
 import { auth } from "@/lib/auth/server";
 import { db, recipes, eq, and } from "@epicure/db";
 import { PrintTrigger } from "@/components/recipe/print-trigger";
+import { hasQuantity } from "@/lib/fractions";
 
 type Params = { params: Promise<{ id: string }> };
 
@@ -109,7 +110,7 @@ export default async function RecipePrintPage({ params }: Params) {
               {recipe.ingredients.map((ing) => (
                 <li key={ing.id}>
                   <span className="qty">
-                    {[ing.quantity, ing.unit].filter(Boolean).join(" ")}
+                    {[hasQuantity(ing.quantity) ? ing.quantity : null, ing.unit].filter(Boolean).join(" ")}
                   </span>
                   {ing.rawName}
                   {ing.note && <span style={{ color: "#888" }}> ({ing.note})</span>}