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/__tests__/grocery-export.test.ts

@@ -0,0 +1,29 @@
+import { describe, it, expect } from "vitest";
+import { buildGroceryExportPayload, groceryExportToText } from "../grocery-export";
+
+describe("buildGroceryExportPayload", () => {
+  it("maps unchecked items and drops checked ones", () => {
+    const payload = buildGroceryExportPayload({
+      name: "Weekly groceries",
+      items: [
+        { rawName: "Milk", quantity: "1", unit: "L", checked: false },
+        { rawName: "Eggs", quantity: "12", unit: null, checked: true },
+      ],
+    });
+    expect(payload.listName).toBe("Weekly groceries");
+    expect(payload.items).toEqual([{ name: "Milk", quantity: "1", unit: "L" }]);
+  });
+});
+
+describe("groceryExportToText", () => {
+  it("renders a plain-text list with quantities", () => {
+    const text = groceryExportToText({
+      listName: "Weekly groceries",
+      items: [
+        { name: "Milk", quantity: "1", unit: "L" },
+        { name: "Bananas", quantity: null, unit: null },
+      ],
+    });
+    expect(text).toBe("Weekly groceries\n\n1 L Milk\nBananas");
+  });
+});