Adds a full shopping list feature to the app so users can manage grocery and other shopping items.

Changes Made

• src/storage.ts: Added ShoppingItem interface (id, name, optional quantity, completed, createdDate), added shoppingList: ShoppingItem[] to AppData, updated createInitialData() and buildMigratedData() to include the new field, and added five CRUD methods: addShoppingItem, updateShoppingItem, deleteShoppingItem, getShoppingItems, and clearCompletedShoppingItems.
• index.html: Added a 🛒 Shopping navigation tab and a full tab section with a shopping list container, empty-state message, and an Add/Edit modal with an item name (required) and quantity (optional) field.
• src/app.ts: Imported ShoppingItem, added currentEditingShoppingItemId state, registered event listeners, wired up shopping in switchTab(), and implemented renderShoppingList, renderShoppingItem, openShoppingItemModal, closeShoppingItemModal, saveShoppingItem, deleteShoppingItem, and clearCompletedShoppingItems methods.
• css/styles.css: Added shopping list styles consistent with the existing design system, including a responsive mobile breakpoint.
• tests/storage.test.ts: Added 10 unit tests covering all CRUD operations for the shopping list.

Features

• Add items with a name and an optional quantity
• Check items off as collected (strikethrough + dimmed styling)
• Edit or delete existing items via a modal
• "Clear Checked" button removes all completed items at once
• Data persisted to localStorage with full migration support for existing data

Testing

• ✅ TypeScript compilation passes
• ✅ All 97 unit tests pass
• ✅ Manual UI verification confirmed

• Fixes Add a shopping list feature #56

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