feat: Feat/recipient onboarding

[Happybello365]

Description
First-time recipients receiving a payment have no guidance on what to do. This issue adds a recipient-specific onboarding flow triggered when a wallet connects and is found to be a recipient on any invoice — explaining what StellarSplit is and how to track their incoming payment.

Technical Context
Involves layout.tsx and a new src/components/RecipientOnboarding.tsx. After wallet connects, check splitClient.getInvoicesByRecipient(). If results exist and recipientOnboarded not in localStorage, show a 3-step modal: (1) you have incoming payments, (2) how release works, (3) go to recipient portal.

Acceptance Criteria
Onboarding triggers for first-time recipients only
3-step modal with progress indicator
Step 1 shows count of invoices they are a recipient on
Step 3 links to /recipient portal page
Completion sets recipientOnboarded in localStorage
Mobile responsive — works on 375px screens
No TypeScript errors
closes #123

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