feat: implement remaining TODO.md feature ideas + fix mobile headers

Implements the six previously-unscoped feature ideas plus a mobile
layout fix reported via screenshot:

- Mobile: Recipes/Collections/Pantry/Meal Plan/Shopping Lists headers
  now stack and wrap instead of clipping buttons on narrow viewports.
- Recipe diff/compare view: word/list diff against any past version,
  next to Restore in version history.
- Shared meal plans & shopping lists: new shoppingListMembers/
  mealPlanMembers tables (viewer/editor roles, mirrors
  collectionMembers), share dialogs, membership-checked routes.
- PDF cookbook export: /print/collection/[id] renders a whole
  collection with page breaks, using the existing print-CSS pattern
  instead of adding a PDF rendering dependency.
- Grocery delivery handoff: shopping lists can copy-as-text (works
  today) or send to Instacart once INSTACART_API_KEY is configured
  (stub adapter — real API needs a partner agreement).
- Personalized "For You" feed tab: ranks public recipes by tag/
  dietary overlap with the user's favorited/highly-rated history.
- PWA: added manifest.json + icons on top of the existing service
  worker so the app is installable; cook-mode pages were already
  cached for offline use.

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

apps/web/app/(app)/collections/[id]/page.tsx

@@ -1,11 +1,15 @@
 import type { Metadata } from "next";
 import { notFound } from "next/navigation";
 import { headers } from "next/headers";
+import Link from "next/link";
+import { Printer } from "lucide-react";
 import { auth } from "@/lib/auth/server";
 import { db, collections, eq, and, or } from "@epicure/db";
 import { RecipeCard } from "@/components/recipe/recipe-card";
 import { ForkCollectionButton } from "@/components/collections/fork-collection-button";
 import { ShareCollectionButton } from "@/components/collections/share-collection-button";
+import { buttonVariants } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
 
 type Params = { params: Promise<{ id: string }> };
 
@@ -30,7 +34,7 @@ export default async function CollectionPage({ params }: Params) {
 
   return (
     <div className="space-y-6">
-      <div className="flex items-start justify-between gap-4">
+      <div className="flex flex-col gap-3 sm:flex-row sm:items-start sm:justify-between">
         <div>
           <h1 className="text-3xl font-bold tracking-tight">{col.name}</h1>
           {col.description && <p className="text-muted-foreground mt-1">{col.description}</p>}
@@ -38,7 +42,13 @@ export default async function CollectionPage({ params }: Params) {
             {col.recipes.length} recipe{col.recipes.length !== 1 ? "s" : ""} · {col.isPublic ? "Public" : "Private"}
           </p>
         </div>
-        <div className="flex items-center gap-2">
+        <div className="flex flex-wrap items-center gap-2">
+          {col.recipes.length > 0 && (
+            <Link href={`/print/collection/${id}`} target="_blank" className={cn(buttonVariants({ variant: "outline", size: "sm" }))}>
+              <Printer className="h-4 w-4" />
+              Export as PDF
+            </Link>
+          )}
           {isOwner && <ShareCollectionButton collectionId={id} />}
           {!isOwner && col.isPublic && (
             <ForkCollectionButton collectionId={id} />