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/messages/en.json

@@ -327,7 +327,8 @@
     "partialMatches": "Partial matches",
     "noMatches": "No strong matches found",
     "missing": "Missing",
-    "ingredientProgress": "{matched}/{total} ingredients"
+    "ingredientProgress": "{matched}/{total} ingredients",
+    "useItUp": "Use it up"
   },
   "mealPlan": {
     "title": "Meal Plan",
@@ -386,6 +387,8 @@
     "following": "Following",
     "trending": "Trending",
     "trendingEmpty": "No trending recipes this week.",
+    "forYou": "For You",
+    "forYouEmpty": "Favorite or rate some recipes to get personalized picks here.",
     "loading": "Loading…"
   },
   "shoppingLists": {

apps/web/messages/fr.json

@@ -315,7 +315,8 @@
     "partialMatches": "Correspondances partielles",
     "noMatches": "Aucune correspondance trouvée",
     "missing": "Manquant",
-    "ingredientProgress": "{matched}/{total} ingrédients"
+    "ingredientProgress": "{matched}/{total} ingrédients",
+    "useItUp": "À utiliser vite"
   },
   "mealPlan": {
     "title": "Planning repas",
@@ -374,6 +375,8 @@
     "following": "Abonnements",
     "trending": "Tendances",
     "trendingEmpty": "Aucune recette tendance cette semaine.",
+    "forYou": "Pour vous",
+    "forYouEmpty": "Ajoutez des favoris ou notez des recettes pour des suggestions personnalisées.",
     "loading": "Chargement…"
   },
   "shoppingLists": {