feat: private invoice access code gate with client-side SHA-256 verification

[snooopdog]

Summary

Displays an access code entry screen (AccessCodeGate) before revealing private invoice details. Verifies codes client-side via SHA-256 against the stored hash before permitting payment submission, persisting the unlocked state in sessionStorage for the session.

• Added AccessCodeGate component with show/hide password toggle and shake animation on invalid attempts
• Client-side SHA-256 hash computation and comparison utilities
• Persists unlocked status in sessionStorage per invoice
• Accessible keyboard navigation and ARIA attributes
• Comprehensive unit tests for hash verification logic and AccessCodeGate

Closes #748
Closes #747
Closes #749
Closes #746

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