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/cooking-mode/cooking-mode.tsx

@@ -6,6 +6,7 @@ import { X, ChevronLeft, ChevronRight, List, Mic, MicOff, Play, Square, HelpCirc
 import { cn } from "@/lib/utils";
 import { useTranslations } from "next-intl";
 import { useLocale } from "@/lib/i18n/provider";
+import { hasQuantity } from "@/lib/fractions";
 
 type Step = { id: string; instruction: string; timerSeconds: number | null };
 type Ingredient = { rawName: string; quantity: string | null; unit: string | null };
@@ -345,7 +346,7 @@ export function CookingMode({
               {ingredients.map((ing, i) => (
                 <li key={i} className="text-sm flex gap-2">
                   <span className="text-muted-foreground shrink-0 w-16 text-right">
-                    {ing.quantity}{ing.unit ? ` ${ing.unit}` : ""}
+                    {hasQuantity(ing.quantity) ? ing.quantity : ""}{ing.unit ? ` ${ing.unit}` : ""}
                   </span>
                   <span>{ing.rawName}</span>
                 </li>