Split `useInvoiceForm` into `useLineItems` and `useInvoiceMeta` sub-hooks

[Kingsman-99]

Description

useInvoiceForm manages both invoice metadata (title, dates, currency, recipients) and line-item state in a single hook, making it over 300 lines and hard to maintain. Splitting it into useLineItems (item CRUD, totals, reordering) and useInvoiceMeta (top-level fields, validation) improves cohesion and testability.

Acceptance Criteria

• src/hooks/useLineItems.ts contains all line-item state and handlers
• src/hooks/useInvoiceMeta.ts contains all invoice-level metadata state and validation
• useInvoiceForm is updated to compose the two sub-hooks and re-export a unified API so callers require no changes
• No existing call site for useInvoiceForm needs to be modified
• Existing tests continue to pass

Context

• Target file: src/hooks/useInvoiceForm.ts