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-export.ts

@@ -0,0 +1,33 @@
+export type GroceryExportItem = {
+  name: string;
+  quantity: string | null;
+  unit: string | null;
+};
+
+export type GroceryExportPayload = {
+  listName: string;
+  items: GroceryExportItem[];
+};
+
+type ShoppingListForExport = {
+  name: string;
+  items: Array<{ rawName: string; quantity: string | null; unit: string | null; checked: boolean }>;
+};
+
+/** Maps a shopping list into a provider-agnostic export shape any grocery-delivery adapter can consume. */
+export function buildGroceryExportPayload(list: ShoppingListForExport): GroceryExportPayload {
+  return {
+    listName: list.name,
+    items: list.items
+      .filter((i) => !i.checked)
+      .map((i) => ({ name: i.rawName, quantity: i.quantity, unit: i.unit })),
+  };
+}
+
+export function groceryExportToText(payload: GroceryExportPayload): string {
+  const lines = payload.items.map((i) => {
+    const qty = [i.quantity, i.unit].filter(Boolean).join(" ");
+    return qty ? `${qty} ${i.name}` : i.name;
+  });
+  return [payload.listName, "", ...lines].join("\n");
+}