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/r/[id]/page.tsx

@@ -11,6 +11,7 @@ import { buttonVariants } from "@/components/ui/button";
 import { Separator } from "@/components/ui/separator";
 import { ServingScaler } from "@/components/recipe/serving-scaler";
 import { getPublicUrl } from "@/lib/storage";
+import { hasQuantity } from "@/lib/fractions";
 import { cn } from "@/lib/utils";
 
 type Params = { params: Promise<{ id: string }> };
@@ -68,7 +69,7 @@ export default async function PublicRecipePage({ params }: Params) {
     prepTime: recipe.prepMins ? `PT${recipe.prepMins}M` : undefined,
     cookTime: recipe.cookMins ? `PT${recipe.cookMins}M` : undefined,
     recipeIngredient: recipe.ingredients.map((i) =>
-      [i.quantity, i.unit, i.rawName].filter(Boolean).join(" ")
+      [hasQuantity(i.quantity) ? i.quantity : null, i.unit, i.rawName].filter(Boolean).join(" ")
     ),
     recipeInstructions: recipe.steps.map((s) => ({
       "@type": "HowToStep",