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/lib/fractions.ts

@@ -36,10 +36,17 @@ export function formatQuantity(value: number): string {
   return whole === 0 ? bestFraction : `${whole} ${bestFraction}`;
 }
 
+export function hasQuantity(quantity: string | null | undefined): boolean {
+  if (!quantity) return false;
+  const raw = parseFloat(quantity);
+  return !isNaN(raw) && raw !== 0;
+}
+
 export function scaleQuantity(baseQuantity: string | null, base: number, desired: number): string {
   if (!baseQuantity) return "";
   const raw = parseFloat(baseQuantity);
   if (isNaN(raw)) return baseQuantity;
+  if (raw === 0) return "";
   const scale = base === 0 ? 1 : desired / base;
   return formatQuantity(raw * scale);
 }