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/for-you-ranking.ts

@@ -0,0 +1,43 @@
+export type TaggedRecipe = {
+  id: string;
+  tags: string[];
+  dietaryTags: Record<string, boolean> | null;
+  createdAt: Date;
+};
+
+/** Collapses a recipe's tags + true dietary-tag keys into one flat tag list. */
+function tagSet(recipe: Pick<TaggedRecipe, "tags" | "dietaryTags">): string[] {
+  const dietary = Object.entries(recipe.dietaryTags ?? {})
+    .filter(([, v]) => v)
+    .map(([k]) => k);
+  return [...recipe.tags, ...dietary];
+}
+
+/** Builds a tag → frequency map from the recipes a user has favorited/highly rated. */
+export function buildPreferenceMap(likedRecipes: Array<Pick<TaggedRecipe, "tags" | "dietaryTags">>): Map<string, number> {
+  const map = new Map<string, number>();
+  for (const recipe of likedRecipes) {
+    for (const tag of tagSet(recipe)) {
+      map.set(tag, (map.get(tag) ?? 0) + 1);
+    }
+  }
+  return map;
+}
+
+/**
+ * Scores a candidate recipe by how many of its tags overlap with the user's
+ * preference map (weighted by how often that tag shows up in their history).
+ * Recency is used only as a tiebreaker (see rankForYou) — an empty preference
+ * map scores everything 0, which the caller should treat as "fall back to
+ * trending" rather than a meaningful ranking.
+ */
+export function scoreCandidate(candidate: Pick<TaggedRecipe, "tags" | "dietaryTags">, preferences: Map<string, number>): number {
+  return tagSet(candidate).reduce((sum, tag) => sum + (preferences.get(tag) ?? 0), 0);
+}
+
+export function rankForYou<T extends TaggedRecipe>(candidates: T[], preferences: Map<string, number>): T[] {
+  return [...candidates]
+    .map((c) => ({ c, score: scoreCandidate(c, preferences) }))
+    .sort((a, b) => b.score - a.score || b.c.createdAt.getTime() - a.c.createdAt.getTime())
+    .map(({ c }) => c);
+}