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/lib/grocery-providers/instacart.ts

@@ -0,0 +1,27 @@
+import type { GroceryExportPayload } from "@/lib/grocery-export";
+
+/**
+ * Stub adapter for the Instacart "Recipe & Shopping List" partner API.
+ *
+ * Not wired to a live endpoint — Instacart requires a signed partnership
+ * agreement and a per-integration API key before any request can succeed.
+ * This documents the request shape so activation is a config change, not a
+ * rewrite, once `INSTACART_API_KEY` is issued.
+ *
+ * Real endpoint (per Instacart Developer Platform docs, subject to change):
+ *   POST https://connect.instacart.com/idp/v1/products/products_link
+ *   Authorization: Bearer <INSTACART_API_KEY>
+ *   Body: { title: string, link_type: "shopping_list", line_items: [{ name, quantity, unit }] }
+ * Response contains a `products_link_url` the user is redirected to.
+ */
+export async function createInstacartShoppingListLink(
+  payload: GroceryExportPayload
+): Promise<{ url: string } | null> {
+  const apiKey = process.env["INSTACART_API_KEY"];
+  if (!apiKey) return null;
+
+  throw new Error(
+    "Instacart integration is stubbed — INSTACART_API_KEY is set but no live API call is wired up yet. " +
+      `Would send list "${payload.listName}" with ${payload.items.length} item(s).`
+  );
+}