Add referral code support to `submit_invoice()`

[Nursca]

Description
A referral system rewards users who onboard new freelancers to the platform. Adding an optional referral code field to submit_invoice() enables tracking and reward distribution without requiring a separate contract.

Requirements and context

• submit_invoice() accepts optional referral_code: Option<BytesN<32>>
• Store referral code on invoice struct
• Emit referral_code in InvoiceSubmitted event (if provided)
• Track referral counts per code in storage
• get_referral_stats(code: BytesN<32>) -> u64 returns total invoices submitted with that code
• Write tests for with and without referral code

Suggested execution

git checkout -b feat/referral-codes

• Add optional field to submit_invoice()
• Track in storage
• Implement stats view
• Write tests

Example commit message
feat: add optional referral code tracking to submit_invoice