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/serving-scaler.tsx

@@ -3,7 +3,7 @@
 import { useState } from "react";
 import { Minus, Plus, Sparkles, X } from "lucide-react";
 import { Button } from "@/components/ui/button";
-import { scaleQuantity } from "@/lib/fractions";
+import { scaleQuantity, hasQuantity } from "@/lib/fractions";
 import { SubstituteIngredientPopover } from "@/components/recipe/substitute-ingredient-popover";
 
 type Ingredient = {
@@ -135,7 +135,7 @@ export function ServingScaler({
               <li key={ing.id} className="flex gap-2 text-sm group">
                 <span className="font-medium tabular-nums min-w-[3rem] text-right">
                   {aiIng
-                    ? `${aiIng.quantity}${aiIng.unit ? ` ${aiIng.unit}` : ""}`
+                    ? `${hasQuantity(aiIng.quantity) ? aiIng.quantity : ""}${aiIng.unit ? ` ${aiIng.unit}` : ""}`
                     : scaled
                       ? `${scaled}${ing.unit ? ` ${ing.unit}` : ""}`
                       : ing.unit ?? ""}