feat(shopping): ShoppingCartStore (Client)

Svelte-5-Runes-Store mit uncheckedCount, recipeIds und loaded.
refresh() holt Snapshot via GET /api/shopping-list, addRecipe/
removeRecipe posten bzw. loeschen und refreshen anschliessend.
Bei Netzwerkfehler bleibt der letzte bekannte State erhalten.

tests/unit/shopping-cart-store.test.ts

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+// @vitest-environment jsdom
+import { describe, it, expect, vi } from 'vitest';
+import { ShoppingCartStore } from '../../src/lib/client/shopping-cart.svelte';
+
+type FetchMock = ReturnType<typeof vi.fn>;
+
+function snapshotBody(opts: {
+  recipeIds?: number[];
+  uncheckedCount?: number;
+}) {
+  return {
+    recipes: (opts.recipeIds ?? []).map((id) => ({
+      recipe_id: id, title: `R${id}`, image_path: null, servings: 4, servings_default: 4
+    })),
+    rows: [],
+    uncheckedCount: opts.uncheckedCount ?? 0
+  };
+}
+
+function makeFetch(responses: unknown[]): FetchMock {
+  const queue = [...responses];
+  return vi.fn(async () => ({
+    ok: true,
+    status: 200,
+    json: async () => queue.shift()
+  } as Response));
+}
+
+describe('ShoppingCartStore', () => {
+  it('refresh populates recipeIds and uncheckedCount', async () => {
+    const fetchImpl = makeFetch([snapshotBody({ recipeIds: [1, 2], uncheckedCount: 3 })]);
+    const store = new ShoppingCartStore(fetchImpl);
+    await store.refresh();
+    expect(store.uncheckedCount).toBe(3);
+    expect(store.isInCart(1)).toBe(true);
+    expect(store.isInCart(2)).toBe(true);
+    expect(store.isInCart(3)).toBe(false);
+    expect(store.loaded).toBe(true);
+  });
+
+  it('addRecipe posts then refreshes', async () => {
+    const fetchImpl = makeFetch([
+      {},
+      snapshotBody({ recipeIds: [42], uncheckedCount: 5 })
+    ]);
+    const store = new ShoppingCartStore(fetchImpl);
+    await store.addRecipe(42);
+    expect(fetchImpl.mock.calls[0][0]).toBe('/api/shopping-list/recipe');
+    expect(fetchImpl.mock.calls[0][1]).toMatchObject({ method: 'POST' });
+    expect(store.isInCart(42)).toBe(true);
+    expect(store.uncheckedCount).toBe(5);
+  });
+
+  it('removeRecipe deletes then refreshes', async () => {
+    const fetchImpl = makeFetch([
+      {},
+      snapshotBody({ recipeIds: [], uncheckedCount: 0 })
+    ]);
+    const store = new ShoppingCartStore(fetchImpl);
+    await store.removeRecipe(42);
+    expect(fetchImpl.mock.calls[0][0]).toBe('/api/shopping-list/recipe/42');
+    expect(fetchImpl.mock.calls[0][1]).toMatchObject({ method: 'DELETE' });
+    expect(store.uncheckedCount).toBe(0);
+  });
+
+  it('refresh keeps last known state on network error', async () => {
+    const fetchImpl = vi.fn().mockRejectedValue(new Error('offline'));
+    const store = new ShoppingCartStore(fetchImpl);
+    store.uncheckedCount = 7;
+    await store.refresh();
+    expect(store.uncheckedCount).toBe(7);
+  });
+});