feat(security): client-side QR code generation for claim links (#409)

[kike-alt]

Closes #409

Confirms and implements client-side QR code generation for claim links for Issue #409.

• Client-Side SVG QR Component: Added pure client-side SVG QR code generator in frontend/components/qr-code.tsx rendering claim link matrices in local browser memory without transmitting URLs to remote APIs.
• UI Integration: Integrated QRCodeModalButton into SharePrompt (frontend/components/share-prompt.tsx).
• Security Requirements: Updated docs/security-model.mdx under Claim Token Security section adding the Client-Side QR Code Rendering Guarantee rule.
• Automated Unit Test: Added frontend/components/qr-code.test.tsx verifying client-side rendering and confirming zero external network calls.

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