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__/meal-plan-access.test.ts

@@ -0,0 +1,59 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+const { mockPlanFindFirst, mockMemberFindFirst } = vi.hoisted(() => ({
+  mockPlanFindFirst: vi.fn(),
+  mockMemberFindFirst: vi.fn(),
+}));
+
+vi.mock("@epicure/db", () => ({
+  db: {
+    query: {
+      mealPlans: { findFirst: mockPlanFindFirst },
+      mealPlanMembers: { findFirst: mockMemberFindFirst },
+    },
+  },
+  mealPlans: { id: "id", userId: "user_id" },
+  mealPlanMembers: { mealPlanId: "meal_plan_id", userId: "user_id" },
+  eq: vi.fn((a, b) => ({ a, b, op: "eq" })),
+  and: vi.fn((...args) => ({ args, op: "and" })),
+}));
+
+const { getMealPlanAccessById, canWriteMealPlan } = await import("../meal-plan-access");
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+describe("getMealPlanAccessById", () => {
+  it("returns null when the plan doesn't exist", async () => {
+    mockPlanFindFirst.mockResolvedValue(undefined);
+    expect(await getMealPlanAccessById("plan-1", "user-1")).toBeNull();
+  });
+
+  it("grants owner role to the plan's userId", async () => {
+    mockPlanFindFirst.mockResolvedValue({ id: "plan-1", userId: "user-1" });
+    const access = await getMealPlanAccessById("plan-1", "user-1");
+    expect(access?.role).toBe("owner");
+  });
+
+  it("grants the member's assigned role", async () => {
+    mockPlanFindFirst.mockResolvedValue({ id: "plan-1", userId: "user-owner" });
+    mockMemberFindFirst.mockResolvedValue({ role: "viewer" });
+    const access = await getMealPlanAccessById("plan-1", "user-2");
+    expect(access?.role).toBe("viewer");
+  });
+
+  it("returns null when the user is neither owner nor a member", async () => {
+    mockPlanFindFirst.mockResolvedValue({ id: "plan-1", userId: "user-owner" });
+    mockMemberFindFirst.mockResolvedValue(undefined);
+    expect(await getMealPlanAccessById("plan-1", "user-2")).toBeNull();
+  });
+});
+
+describe("canWriteMealPlan", () => {
+  it("allows owner and editor, denies viewer", () => {
+    expect(canWriteMealPlan("owner")).toBe(true);
+    expect(canWriteMealPlan("editor")).toBe(true);
+    expect(canWriteMealPlan("viewer")).toBe(false);
+  });
+});