feat: copy/export as Markdown wherever print exists

Added a shared ExportMarkdownButton (copy to clipboard / download .md)
next to every existing print button: recipe, shopping list,
collection, meal plan, pantry. Each surface gets a small serializer in
lib/markdown/ built from data already in scope at that page — no new
queries except pantry, where items now thread through as a prop to
PantryPageHeader instead of being fetched only for PantryManager.

Also fixes an unrelated bug hit while verifying the collection export:
RecipeCard called the client-only useTranslations() hook without
"use client", so it rendered fine everywhere it happened to run inside
an already-client tree but 500'd — "Couldn't find next-intl config
file" — when Next tried to run it as a Server Component, which only
happens on the collection detail page (its only caller). Collections
with recipes in them were completely broken.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/messages/en.json

@@ -301,6 +301,11 @@
     "save": "Save",
     "saved": "Saved",
     "saveFailed": "Failed to save",
+    "exportMarkdown": "Export as Markdown",
+    "copyMarkdown": "Copy as Markdown",
+    "downloadMarkdown": "Download as Markdown",
+    "copiedToClipboard": "Copied to clipboard",
+    "copyFailed": "Failed to copy",
     "print": "Print",
     "share": "Share",
     "deleteFailed": "Delete failed",