docs: add architecture diagrams

[Adamchaua]

🚀 Work Completed

Added a version-controlled Mermaid architecture page for Haven and linked it from the GitBook sidebar.

🔍 Requirement Match

• Core objective: document the relationships between Haven_Contracts, Haven_Frontend, and Haven_Docs.
• Target files: architecture/system-diagrams.md, SUMMARY.md.
• Scope: documentation-only change for issue docs: add architecture diagram #5.

💻 Changes Included

• Added high-level repository/system architecture diagram.
• Added frontend data-flow sequence for local IMEI hashing, Freighter signing, and Soroban storage.
• Added bounty lifecycle state diagram: register -> report stolen -> killswitch -> escrow lock -> finder claim -> owner confirm -> USDC release.
• Added USDC escrow sequence diagram.
• Added insurance claim flow diagram.
• Linked the page under System Architecture in SUMMARY.md.

🧪 Verification

• git diff --check
• Documentation-only change; no runtime tests required.

🎯 Submission Notes

• Diagrams are Mermaid, not raster images, so they remain reviewable in Git.
• No dependencies added.
• Ready for maintainer review.

If the result saves review time or exceeds expectations, an optional tip is appreciated.

[Adamchaua]

Hi maintainers — adding a short review note here so the PR is easier to triage. This docs PR is meant to make the architecture easier to understand at a glance by adding diagrams rather than changing any implementation behavior. If you prefer a different diagram style, naming convention, or placement inside the docs tree, I can adjust it quickly.