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>

.env.example

@@ -66,3 +66,7 @@ OPENROUTER_DEFAULT_MODEL=google/gemini-flash-1.5
 OPENAI_API_KEY=
 ANTHROPIC_API_KEY=
 OLLAMA_BASE_URL=http://localhost:11434
+
+# Grocery delivery handoff (optional — without these, shopping lists only offer "copy as text")
+NEXT_PUBLIC_GROCERY_PROVIDER=
+INSTACART_API_KEY=