feat: one-click shopping list from meal plan, fix ingredient merge bug

- Meal-plan page now has a direct "Add to shopping list" action for the
  currently-viewed week, instead of requiring a trip to Shopping Lists and
  manually typing the week's Monday date.
- Fixed a real under-shopping bug in shopping-lists/route.ts: when
  generating from a meal-plan week, ingredients appearing in more than one
  recipe were merged by keeping only the FIRST occurrence's quantity and
  silently dropping the rest. New mergeIngredients() (pantry-shopping-match.ts)
  groups by ingredientId (or normalized name+unit) and sums quantities
  across every recipe in the week; incompatible/unparseable units are kept
  as separate line items rather than guessed at.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/pantry-shopping-match.ts

@@ -33,17 +33,17 @@ export type PantryAdjustedItem = {
   inPantry: boolean;
 };
 
-function normalizeName(name: string): string {
+export function normalizeName(name: string): string {
   const trimmed = name.toLowerCase().trim().replace(/\s+/g, " ");
   // Simple plural trim — strip a single trailing "s" for words longer than 3 chars.
   return trimmed.length > 3 && trimmed.endsWith("s") ? trimmed.slice(0, -1) : trimmed;
 }
 
-function normalizeUnit(unit: string | null | undefined): string {
+export function normalizeUnit(unit: string | null | undefined): string {
   return (unit ?? "").toLowerCase().trim();
 }
 
-function formatQuantity(n: number): string {
+export function formatQuantity(n: number): string {
   return Number.isInteger(n) ? String(n) : n.toFixed(2).replace(/\.?0+$/, "");
 }
 
@@ -108,3 +108,60 @@ export function applyPantryToItems(
     return { rawName: item.rawName, quantity: item.quantity, unit: item.unit, inPantry: true };
   });
 }
+
+export type MergeSourceIngredient = {
+  rawName: string;
+  quantity: string | null;
+  unit: string | null;
+  ingredientId: string | null;
+};
+
+export type MergedIngredient = {
+  rawName: string;
+  quantity?: string;
+  unit?: string;
+  ingredientId: string | null;
+};
+
+/**
+ * Merges ingredients across recipes (e.g. an entire week's meal plan) so a shopping
+ * list doesn't show the same ingredient once per recipe. Groups by normalized
+ * ingredientId (if present) or normalized name+unit, then SUMS quantities across the
+ * group — the earlier version of this logic kept only the first occurrence and
+ * silently dropped every other recipe's requirement, which under-shopped whenever the
+ * same ingredient appeared in more than one recipe.
+ *
+ * Items with the same name but incompatible/unparseable units are kept as separate
+ * line items rather than guessed at — never invent a converted total.
+ */
+export function mergeIngredients(ingredients: MergeSourceIngredient[]): MergedIngredient[] {
+  const groups = new Map<string, { rawName: string; ingredientId: string | null; unit: string | null; entries: (string | null)[] }>();
+
+  for (const ing of ingredients) {
+    const unitKey = normalizeUnit(ing.unit);
+    const groupKey = ing.ingredientId
+      ? `id:${ing.ingredientId}:${unitKey}`
+      : `name:${normalizeName(ing.rawName)}:${unitKey}`;
+
+    let group = groups.get(groupKey);
+    if (!group) {
+      group = { rawName: ing.rawName, ingredientId: ing.ingredientId, unit: ing.unit, entries: [] };
+      groups.set(groupKey, group);
+    }
+    group.entries.push(ing.quantity);
+  }
+
+  return Array.from(groups.values()).map((group) => {
+    const parsed = group.entries.map((q) => (q ? parseFloat(q) : NaN));
+    const allParseable = parsed.length > 0 && parsed.every((n) => !isNaN(n));
+
+    if (allParseable) {
+      const total = parsed.reduce((sum, n) => sum + n, 0);
+      return { rawName: group.rawName, quantity: formatQuantity(total), unit: group.unit ?? undefined, ingredientId: group.ingredientId };
+    }
+
+    // Unparseable or mixed with/without quantity — can't sum reliably, don't guess.
+    // Fall back to no quantity shown rather than an incorrect total.
+    return { rawName: group.rawName, quantity: undefined, unit: group.unit ?? undefined, ingredientId: group.ingredientId };
+  });
+}