Check for duplicate email/phone when adding a new customer

[kilbot]

Feature Request

When a cashier creates a new customer from the POS, the system should proactively check for existing customers with the same email address or phone number before saving, in order to prevent duplicate customer records.

Problem

Currently it's easy to accidentally create duplicate customers if a cashier doesn't know the customer already exists. Duplicate records cause confusion in order history, loyalty tracking, and reporting.

Proposed Solution

• While the cashier fills in the "Add Customer" form, run a live (debounced) lookup against existing customers as they type into the email and phone fields.
• If a match is found, surface a warning/suggestion — e.g. "A customer with this email already exists: Jane Doe. Would you like to use this customer instead?"
• Allow the cashier to either select the existing customer or proceed to create a new one (in case the match is a false positive).

Acceptance Criteria

• Duplicate check fires on email field blur / after a short debounce while typing.
• Duplicate check fires on phone field blur / after a short debounce while typing.
• UI clearly surfaces any matching customer(s) with name and identifier.
• Cashier can choose to switch to the existing customer or dismiss the warning and continue creating a new one.
• No duplicate check is performed if the field is empty.