docs: transaction lifecycle architecture diagram

[nanaf6203-bit]

Summary

There is no diagrammatic summary of the transaction lifecycle from sign through confirm. New contributors have to grep through wallet, blockchainSecurity, transactionService, and store code to understand the flow.

Affected area

• docs/ (no end-to-end diagram)
• Conceptual coverage of: src/utils/eip712/eip712Signing.ts, src/hooks/useSecureTransaction.ts, src/lib/transactionService.ts, src/store/transactionStore.ts

Acceptance criteria

• Add a Mermaid sequence diagram covering: sign → submit → confirm → cache → audit.
• Place it under docs/transaction-lifecycle.md.
• Cross-link from the README onboarding section.
• Validate the diagram renders correctly on GitHub PR preview.

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Filed automatically as part of the PropChain code-review sweep.