Handle adding duplicate items: allow separate item for unpacked, merge when packed

[dralan71]

Originally issue #6 in dralan71/luggage-tracker

Overview

Currently, when adding a new item (via quick add or add item), if the item already exists, the quantity simply increases. However, to improve usability:

• If the newly added item matches an existing item that is already packed, it should be added as a separate unpacked item.
• When the new unpacked item is marked as packed, it should automatically merge with the existing packed item, updating the quantity accordingly and leaving only one packed item card.

Example:

• You have "socks" (quantity: 5, packed) in your list.
• You use quick add to add another "socks" item (quantity: 1, unpacked by default).
• The new "socks" item appears as a separate unpacked card.
• After physically packing the new socks and toggling them as packed in the app, the two "socks" items should merge into one card with quantity 6, packed.

Tasks

• Update the item addition logic to allow duplicate items as separate cards if one is already packed.
• Implement merging logic so that when an unpacked duplicate is marked as packed, it merges with the already packed item.
• Ensure the UI reflects these changes clearly and intuitively.
• Test for edge cases, such as bulk adds and rapid toggling between packed/unpacked.

Expected Outcome

Users can add more of an item even if it is already packed, and merging occurs automatically upon packing, streamlining item management.