Add invoice chat panel with localStorage-backed off-chain messaging

[lemarjohnny781]

Adds a simple off-chain invoice chat panel to the invoice detail page.

Introduces InvoiceChat.tsx
Stores invoice messages in localStorage under invoice-chat-{invoiceId}
Allows creator and recipients to exchange messages visible to all invoice parties
Displays sender as truncated wallet address and messages in chronological order
Disables chat input for wallets not listed on the invoice
Preserves mobile responsiveness for narrow screens

This feature provides lightweight in-app communication for invoice participants without requiring backend storage.

closes #107

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