feat: N-of-M co-signer approvals UI, invitation flow, and real-time status page

[meshmesh001]

Summary

Implements the multi-signature N-of-M co-signer workflow, enabling creators to configure co-signers and thresholds during invoice creation, and provides a dedicated approval tracking page (/invoice/[id]/cosigners) with real-time updates and wallet approval actions.

• Added CoSignerSection to invoice creation options and review steps
• Created /invoice/[id]/cosigners with real-time status via useInvoiceStream, threshold progress bar, and list of required co-signers
• Enabled connected Freighter wallets to submit co-signer approvals directly from the page
• Added share co-signer link with QR code modal and copy button
• Added unit tests for co-signer status and threshold logic

Closes #741
Closes #740
Closes #739
Closes #738

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